How do you like your steak cooked? There is only one right way to interpret the doneness you like---cut the steak in half and check it. That's not practical for a chef to do when cooking in a restaurant. Steaks may vary in thickness and size, and the grill may vary in temperature. Also the temperature of the meat in the refrigerator may vary by a few degrees. All of these factors contribute to how much time the steak spends on the grill and the end result. An experienced chef, after years of practice, can tell the doneness just by feeling the firmness of the steak, rare being soft and spongy and well done being very firm.
The only sure way of determining the doneness is to test the internal temperature with an instant read thermometer inserted precisely into the center of the meat at a 45 degree angle to the flat surface. This will ensure a correct reading of the internal temperature. Don't worry about losing any juices---it's only one small hole you're making. The pictures below represent what each doneness means to a chef.
Black and Blue
Internal Temp. 70 F
Black and Blue---Seared black on outside, ice cold raw center and will be slippery soft in texture.
Internal Temp. 100 F
Very Rare---Hot on the outside, raw on the inside and will be sort of wobbly.
Internal Temp. 120 F
Rare---Red cool to warm center and will be soft and spongy.
Internal Temp. 126 F
Medium Rare---Red warm center and will have a springy firmness.
Internal Temp. 135 F
Medium---Hot pink center and will have a less springy firmness than medium rare.
Internal Temp. 145 F
Medium Well---Slight color, cooked throughout and will feel firm.
Internal Temp. 160 F
Well Done---No color Left and will feel very firm and unyielding.