Conjunctions and oppositions are easy aspects to spot. The square aspect is a little trickier. A square aspect occurs between two planets that are 90 degrees apart (within acceptable orb). This basically translates to planets that are in the same mode, i.e. cardinal, fixed, or mutable, but not in the same sign and not in opposite signs.
Consider the cardinal mode, for instance. The signs in this mode (cardinal signs) are Aries, Cancer, Libra, and Capricorn. If a planet is in Aries, it would be square another planet in either Cancer or Capricorn (different than Aries), but not Libra (since it is opposite Aries, and would fall under the opposition aspect). Of course, the orb has to be of acceptable limit. So a planet at 1 degree of Aries would not be square a planet in 28 degrees of Capricorn. Taking the shorter arc, Capricorn-Aries, (explained in the post on aspects), the separation between the planets would be 2 degrees of Capricorn + 30 degrees of Aquarius + 30 degrees of Pisces + 1 degree of Aries = 63 degrees, which is well short of the 90 degrees requirement.
In the fixed mode, the signs Taurus and Scorpio are square the signs Leo and Aquarius, and vice versa. And in the mutable mode, Gemini and Sagittarius are square the signs Virgo and Pisces, and vice versa.
Squares can be spotted in the Ephemeris, but it takes more doing than spotting a conjunction or opposition. It is a lot easier if you lay the planets out in a wheel, where the squaring signs are easier to spot, especially because you know to look for a 90 degree separation (or right angles). Here are the planets in Steve Jobs’s horoscope, laid out in a wheel:

The Moon position is accurate, since I have used his place of birth as well as exact time to compute all positions. (The difference between ephemeris positions and actual positions, particulary in regard to the Moon’s position, is explained in the Ephemeris – Universal Time post).
Let’s identify the squares. Consider the cardinal mode first – this would have us look at Aries-Cancer-Libra-Capricorn, or the 1st-4th-7th-10th signs. Which are easy to spot in the wheel. You see Moon and Mars in Aries, Jupiter and Uranus in Cancer, Neptune in Libra, and Venus in Capricorn. (If you are not yet sure about your reading of the symbols for the planets and signs, see the bottom of the Ephemeris post for a translation from symbol/glyph to name.) These are the squares in the cardinal mode within acceptable orb limits:

The Mars-Uranus square has an orb of 5 degrees. The Mars-Jupiter square is a little wider, with an orb of just under 9 degrees. You may balk at including this, but because Uranus is already squaring Mars, it pulls the neighboring Jupiter into the mix. Next, Mars is square Venus with an orb of under 8 degrees. And Venus is square Neptune with an orb of under 7 degrees. Neptune is square Uranus and Jupiter, but we don’t consider these since these squares would be generational, given the long orbital cycles of the planets involved.
What about the Moon, does it make any squares? The closest candidate is Jupiter, but even that would be an orb of about 13 degrees, which is outside of a special acceptable orb of 10-12 degrees that we might assign to the Moon, since it is so vital to life.
Next, let’s consider the fixed mode signs: Taurus-Leo-Scorpio-Aquarius, which are the 2nd-5th-8th-11th signs. Pluto is in Leo, Saturn is in Scorpio, and Mercury is in Aquarius. There’s a Mercury-Saturn square for sure, with an orb of under 7 degrees:

There is also a Saturn-Pluto square, but again, we ignore it because it is a generational aspect.
Lastly, let’s take a look at the mutable mode signs: Gemini-Virgo-Sagittarius-Pisces which are the 3rd-6th-9th-12th signs. The Sun in Pisces is the only presence in the mutable mode, so there are no squares.
In sum, Jobs’s horoscope has five squares. This is actually a fairly large number, and is a primary contributor to Jobs’ hard-driving energy and entrepreneurial spirit.
The square is considered an aspect of challenge: planets in square to each other egg each other on to do something! Sitting on your hands is not an option, because the friction between the planets in a square demands resolution. It’s like a pebble in your shoe. The challenge posed by a square shows up as obstacles and frustration, and action must be taken to surmount the challenge. Success is not guaranteed, but if you don’t take action, the square will slowly consume you.
Why are squares challenging? The reason is signs in a square are of different elements, which generally don’t see eye to eye. For instance, consider the Aries-Capricorn square. Aries is a fire sign, Capricorn is an earth sign. A planet in Aries wants to be independent, and needs to be left alone so it can engage in adventure and pioneering activities. It is not at all concerned with reputation or standing, and is not necessarily practical. Capricorn, on the other hand, is very much practical and institutional. It likes to work within societal limits and requirements, and is very much concerned with reputation. So Aries and Capricorn essentially point in different directions: your challenge, if you have an Aries-Capricorn square is to somehow make them see eye-to-eye; you simply can’t indulge fully in one aspect because the other aspect will make its presence felt and demand satisfaction.
Since these signs are in cardinal mode, which is about initiating action, the square tends toward entrepreneurship, especially in business. The reason is the pioneering, independent aspect of entrepreneurship satisfies the Aries need, and building something concrete–a business–out of this satisfies Capricorn. It’s not easy, but the rewards of a successful entrepreneurship are significant. Jobs’s Mars-Jupiter/Uranus in Cancer square doubles down on the Aries-Capricorn square, and is even more potent because Jupiter and Uranus bring a mighty dose of energy compared to Venus (the other planet squaring Mars), and the nature of Jupiter and Uranus expand and intensify the Mars energy.
What about the Mercury-Saturn fixed square? The fixed mode is about maintaining and sustaining. Mercury is thought and communication, Saturn is restraint, perseverance, practical knowhow, common sense etc. So the square’s challenge is for Mercury to think and communicate in a sensible manner, sustaining a message in a down-to-earth practical manner that makes sense to its recipients. The challenge here is also between the objective, abstract, idealistic nature of airy Aquarius, and the emotional, instinctive nature of watery Scorpio. If Mercury tends to go on on airy flights of fancy, Saturn will bring it back to earth and make sure the ideas are grounded and appeal to the emotions. You can see for yourself how Steve Jobs worked this square in his famous product launch demos.
All squares are about challenges. But every square brings its own particular type of challenge, depending on the mode of the square (cardinal, fixed, or mutable), and the planets involved.
Go ahead and see what squares, if any, occur in your horoscope. Only consider squares in which at least one of the planets is the Sun, Moon, Mercury, Venus, and Mars; ignore all others. Try not to use an orb of over 10 degrees, unless you happen to have only one square–even then, I wouldn’t go over 12 degrees. (The special case is when you have two or three planets conjunct, and one of them makes a close square, in which you can pull the other conjunct planets into the square as well, as in the Mars-square-Uranus/Jupiter in Jobs’s horoscope.)
You may want to take a screenshot of the empty wheel below, then print it and write in the planets in your horoscope at the appropriate places in the wheel so you can identify the squares without difficulty.

Take up each square in turn, see which mode it is in, then using the keywords of the planets involved (see my posts on the planets for keywords), see how the challenge posed by the planets in square to each other has manifested in your life. If it is a cardinal square, think in terms of challenges in getting things off the ground. If in fixed mode, challenges in terms of a sustained course of action. And if in mutable mode, challenges in terms of changes you want to make, and staying flexible and open to options.