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 finger tips. 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). [...]

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.