Doneness is a tricky thing, and it takes a while to get the hang of. There are two ways to do it.
You can use a meat thermometer:
Insert the probe at a diagonal so the juices don’t run out. Push the tip of the probe to the thickest part of the meat and wait 10-20 seconds.
Make sure you calibrate the thermometer when you drop it, and from time to time, by measuring boiling water. Should read 212F.
I find this a hassle, and use the cook’s best friend, my hands.
You can use your fingers
I push on the meat to find out how much give it has.
How much give should it have?
Put your thumb and index finger together, like so, loosely, don’t press them together:
Feel the fleshy pad under your thumb.
Should have a lot of give. That’s rare. Bloody. Practically tartare.
Now put your thumb and middle finger together, again loosely, like so:
Feel the fleshy pad again.
This is medium rare.
Good for steaks. (Good for burgers too, but that is technically risky.)
Switch to the fourth finger.
This is medium, medium-well.
Just a bit of give. This is good for chicken and pork.
Switch to your pinky.
This is well done. This is good for nothing.
You just killed your meat, and it was already dead.
By the way, I realize hand modeling is not in my future. That dream died a while ago. Please don’t mention it. I’m still bitter.