COOK IT.
Cut into the steak in the center. Rare meat will be red; medium rare will have a little red in the center turning to pink; medium will have some pink in the center; medium well will have a mostly gray or brown color.
Use the “touch test.” Rare will feel soft and only a little springy, with very little resistance. Medium rare will feel firm and springy, with some resistance against your finger. Medium/medium well will be firm and will snap back when pressed.
Use your meat probe. The USDA recommends that you cook steaks to a minimum interior temperature of 145°F (63°C). If you do so, your meat will be gray to brown and about as tender as a bicycle tire. The reality is that you risk getting sick from steaks cooked to lower interior temperatures. I enjoy rare steak enough to risk it, but you may not.