There are two basic methods to test for how done your meat is while you are cooking it—use a meat thermometer, or press on the meat with your fingertips.
The problem with the meat thermometer approach is that when you poke a hole into the meat with a thermometer, it can let juices escape, juices that you would rather have stay in the meat. For this reason, most experienced cooks rely on a "finger test" method, especially on steaks (whole roasts are better tested with a thermometer).
My mother has been trying to get me to test meat with my fingertips for years, and for years, being somewhat of a scaredy cat (won't it burn my fingers?) I ignored, avoided, ran away from the idea.
Then my friend David showed me up. Here's a guy who loves to grill but doesn't know how to boil water. (Really. Cannot boil water. Just ask him, he's proud of the fact.) David taught me how to test for the doneness of meat using this method and these days half the time I don't even bother with a thermometer.
Now the point of this story is not to embarrass David (though that would be fun, if it were even possible) but to encourage you, if like me, you've been shying away from trying this approach. This really isn't rocket science.
This is one of those things that gets easier with practice. The next time you cook a steak, even if you are still planning to rely on a meat thermometer, press on the meat here and there while it cooks, and compare the feeling of the meat with the following finger test. With practice, you will become more confident.
The Finger Test to Check the Doneness of Meat
Open the palm of your hand. Relax the hand. Take the index finger of your other hand and push on the fleshy area between the thumb and the base of the palm. Make sure your hand is relaxed. This is what raw meat feels like. (Check this out the next time you have a raw steak to cook.)
Now gently press the tip of your pinky and your thumb together. Again feel the fleshy area below the thumb. It should feel quite firm. This is what well done meat feels like when you press on it. (Check this out the next time you overcook a piece of meat.)
Press the tip of your ring finger and your thumb together. The flesh beneath the thumb should give a little more. This is what meat cooked to a medium doneness feels like.
Gently press the tip of your middle finger to the tip of your thumb. This is medium rare.
Press the tip of your index finger to the tip of your thumb. The fleshy area below the thumb should give quite a bit. This is what meat cooked to rare feels like. Open up your palm again and compare raw to rare.