I have 2 answers!
First, an instant read electronic meat thermometer is my standby, but it does require breaking the surface - less obtrusive than a knife cut, but still a break!
Second is a trick I picked up somewhere along the way - it's best for thick steaks, but it should still work for anything thicker than a minute steak.
It sounds silly, but it works: Use your index finger and gently press against your forehead; then against the fleshy part of your chin; then against the tip of your nose.
If you press your finger against a cooked steak:
Well-done feels like your forehead
Medium feels like your chin
Rare feels like your nose
Weird but true :D