There are two ways to tell if meat is done. The first is the temperature test, which is good for large roasts but not for steaks; poking steak with a thermometer lets juices escape, making it less tender. The touch test, on the other hand, is what professional chefs most commonly use.
RAW: Hold out your relaxed hand, palm up. Poke the fleshy part of your palm just under your thumb with the index finger of your opposite hand. That should be similar to what raw meat feels like.
RARE: Touch your index finger and thumb together. The fleshy part of your palm should tense up a bit. Feeling this with your opposite index finger will indicate what a steak cooked rare should feel like.
MEDIUM RARE: Touch your thumb and middle finger together. This approximates what medium-rare meat feels like.
Continue touching your thumb to remaining fingers to understand the gradient of what your meat should feel like. By the time you touch your thumb to your pinkie, your meat is well done.