To check if they are root bound you slide the plant out of the cup or pot and check the roots. If they look like they've been circling around at the bottom for a while then they need to be transplanted.

You usually wait until they are all the way dry to check because it'll slide it much easier. To get them out of the pot you just turn it upside down with your hand on top where the plants stalk is at. It should just slide out into your hand. If its hard to get out push on the sides of the pot to loosen it up.

This is what it looks like!