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).
(Via Simply Recipes.)
This has been my method of testing for years. The examples and pictures show the technique well.