Using a thermometer is the best way to tell how done your steak is, but sans thermometer, you can use this touch trick, comparing how the steak feels when you press it to different parts of your face.
Photo by Meng He.
In The Connoisseur's Guide to Meat, Jennie Milsom describes the cheek, chin and forehead test:
An accurate way to test for doneness is to press the steak lightly with your fingertip and compare the way it feels to your cheek, chin and forehead. A rare steak is soft and fleshy like your cheek; fleshy with some resistance, like your chin, is medium; firmer to the touch with more resistance, like your forehead, means it is well done.
This method is similar to the finger test, in which you compare the steak's elasticity with your thumb pad as you touch different fingers. I find it's easier to tell greater degrees of variation when poking my face rather than my palm, but pick the method that works best for you.
The Connoisseur's Guide to Meat [Google Books]