Monitor in two passes, starting with a quick look-over in cupped hands for bleeding, breathing and obvious injury, then a watch over the rest of the day and the following evening for how he moves, eats and uses his wheel. A hamster who is bright, moving evenly, eating and active at his usual hour has likely escaped with a fright; any single red-flag sign, at any point, turns watching into a same-day vet call.
The first five minutes
Scoop him into cupped hands low over a soft surface so a wriggle cannot cause a second fall, and let him sit while you look. Check his breathing is even rather than laboured, look for blood around the nose and mouth, and run your eyes over his legs as he walks across your hands — you are looking for a limb being dragged, held up or used reluctantly. Check the front teeth if he will let you lift a lip gently: teeth broken in a fall are common and easy to miss in the first panic. Offer a favourite food; a hamster who pouches it eagerly is giving you useful information.
Making the cage safe for observation
Put him back into his cage with the high platforms and tall climbing routes removed for now, the wheel taken out for the first hours so he rests rather than testing an injury at full sprint, and food and water placed where he can reach both without climbing. Deep, soft bedding cushions any unsteadiness. Keep the room quiet and resist checking every ten minutes — a hamster who is repeatedly woken and handled cannot show you his normal behaviour, which is the very thing you are trying to judge.
What to watch across the day and evening
Hamsters keep their real hours at night, so the evening tells you the truth. Watch for him emerging at his usual time, moving with an even gait, grooming both sides of his face, eating and drinking normally, and running or exploring when the wheel goes back in. Note anything that is off in specifics — circling, a head tilt, one cheek pouch unused, food dropped repeatedly, squeaking when picked up, or sleeping through his active hours. Vague worry is hard to act on; "he has not touched his food and is dragging his left back leg" is a sentence a vet can act on immediately.
The signs that mean vet today, not tomorrow
Ring the vet the same day — an exotics-aware vet if you have one — if you see bleeding that does not stop, laboured or noisy breathing, a limb dragged or not used, loss of balance or a head tilt, teeth clearly broken, refusal of all food into the evening, or a hamster who is limp, cold or unresponsive at any moment; that last group is an emergency now, not an appointment later. Small animals deteriorate quickly, and a vet would always rather examine a hamster who turns out to be bruised than meet one a day after an injury became something worse.
Questions that come up while you watch
In the anxious hour after a fall, most owners are really asking one question at a time: is this normal, should I wait, should I ring? For one of those worries, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.