Run your hand down his leg and if you feel any heat, rinse his leg of with cold water.
You have to go with your gut, if they look swollen, you’re probably right :)
I do a quick check of my horse's leg every time I ride, feel his tendons and run it along his leg. This way, you can tell if something is wrong. Also rinsing their legs with cold water after a particularly vigorous lesson is very helpful as well to keep the swelling down. If you aren't sure and you think something is wrong, you can always call a vet to make sure.