Experience
Cooking time will vary on the thickness of the steak and the amount of heat. "Push" the meat gently during cooking - some use their finger, others a utensil. It's not necessary to actually break the seal of the meat (you DID sear it didn't you?!) as you're looking for firmness.
Softer = rarer.
Firmer = more done.
The next time you cook a steak, poke it with your finger when it's still raw and observe how much give there is. Poke it again when you take it off the grill or stove; it will be firmer. Experience will let you know how much "give" you want to be in a steak before you take it off the grill.