This technique was taught to me by my momma.
Make a fist. Your hand should be comfortable, not tightly squeezed. Press on the fleshy part between your thumb and pointer finger. There should be plenty of give. This represents what rare meat feels like if you poke it with your finger.
By squeezing a little bit tighter, the amount of give should be a little less. This would represent something closer to medium.
If you squeeze your fist as tight as you can and press on the same spot, there should be little to no give. Your flesh should feel firm. This represents what well done meat would feel like.
Pretty neat huh? This will keep you from guessing and save you from cutting into the meat.