How do you tell when a steak is done?

An instant-read thermometer inserted into the thickest part, away from bone and fat, is the only reliable method, and it costs less than a single ruined steak. Target internal temperatures at the center: about 125 for rare, 130 to 135 for medium rare, 135 to 145 for medium, and above that you are into well done, where the muscle fibers have contracted enough to squeeze out most of the moisture. Pull the steak roughly 5 degrees below your target, because carryover cooking continues to raise the internal temperature while it rests, more so on thick cuts and after high-heat searing.

Resting matters and the usual explanation for why is contested. The traditional claim is that juices redistribute; the better-supported version is that a cooling steak loses less liquid when cut because the muscle fibers relax and the internal pressure drops. Either way, five to ten minutes for a steak makes a visible difference on the cutting board. Two techniques worth adopting: dry the surface thoroughly before searing, since surface moisture must evaporate before browning can begin, and salt either right before cooking or at least 40 minutes ahead, because the window in between is when salt has drawn moisture to the surface without yet being reabsorbed. The finger-poke comparison to parts of your hand is a real technique used by cooks with thousands of repetitions behind it, and it is not something to rely on for a cut you paid for.