After opening you ask: is it there, is it square, is it steerable? How the control check works and why the decision altitude of 2,500 ft matters.
The honest answer up front: the control check is a short function test of your canopy right after opening, with which you find out within seconds whether it flies, brakes and steers [1]. You are not checking the weather or the terrain, you are checking whether your parachute is good enough to land with.
The USPA gives you three questions to remember: Is it there? Is it square? Is it steerable? [1] If you answer yes to all three, you fly on to the landing area. If not, you have a clear decision limit: by 2,500 ft (about 760 m) at the latest you must know whether the canopy is safe to land [1][2]. If it is not, you start your emergency procedure immediately [1].
This guide describes what you learn and why. It does not replace training. You practise the emergency procedures themselves at your school in the harness trainer, not from a text.
After the pull you count "1000-2000-3000" and then check the canopy [3]. The USPA first describes the visual check: is the canopy there (large and fully inflated)? [1] Milestone: you look up before you touch anything.
A good canopy has four clearly visible edges and a rectangular shape. The lines run in four neat groups to the risers, and the slider (the piece of fabric that slows the opening) sits down at the risers [1]. It also flies stable: it does not turn or spin but glides toward the horizon [1]. Fixable problems (such as twisted lines) you deal with first, as you learned in training [1].
Now comes the actual control check. The USPA describes the order like this [1]:
Milestone: you can show the four steps in the right order in the hanging harness on the ground.
The decision altitude for students and A-licence holders is 2,500 ft, for B and C licences 2,000 ft [2]. 2,500 ft is almost five Cologne Cathedrals (157 m each) stacked up. By then you decide: canopy landable or emergency procedure [1]. "If in doubt" means the emergency procedure, because the USPA explicitly advises against trying to land a compromised parachute [1].
Only when the canopy checks out do you orient yourself: check altitude, position over the ground, find the landing area and fly to the holding area [1].
This explains and motivates, it does not replace training. Emergency procedures are sequences that your body must know faster than your head, and they are practised at school.
Pay attention to the feel: when you pull the toggles all the way down, the parachute should become noticeably slower. Exactly this feeling is what you need later when landing. The USPA names raising the toggles out of the flare as part of the test [1]. So you do not only test whether the canopy brakes, you also learn how it feels in a flare.
The control check is the physical part of the canopy check after opening: you release the toggles, check straight flight, brake with a full flare and turn at least 90° right and left [1]. That way you know the parachute steers and brakes.
The USPA names: Is it there? Is it square? Is it steerable? [1] In the DFV manual the canopy check follows the count "1000-2000-3000" [3]. Both systems teach the same order: see first, then test.
For students and A-licence holders the USPA names 2,500 ft as the decision altitude, for B and C 2,000 ft [2]. If the canopy is not safely landable by then, you start the emergency procedure immediately [1]. Your school may set different heights.
Twisted lines, closed end cells, a slider that stays up or an unintended turn are usually classed as minor problems by the USPA, which you try to fix first [1]. If the problem gets worse, you start the emergency procedure by 2,500 ft [1]. What is right in each case you clarify in harness training.
Toggle Turns: Steering Your Parachute With the Steering Lines