Myth: Stabbing the Meat with a Thermometer or a Fork Will Drain it of Vital Juices but Cutting it is a Good Way to Tell if it is Done
Yet another myth: Using a thermometer will cause your meat to bleed out, and you must never use a fork to turn your meat. But cutting into your meat will help you tell when it is done.
There is no seal or moisture barrier on the surface of your meat. Water gets out by dripping and evaporating all through the cook. Searing meat is done to develop complex flavors, not to seal it. So poking a hole in it doesn't destroy its integrity. Meat is not a balloon. It won't go pfffffft and deflate.
An eight ounce steak is 75% water. That's six ounces of water. That's about 36 teaspoons. If you stick a thermometer probe into the meat at most you'll lose 1/4 teaspoon. Let's say you treat it like a pin cushion and you lose a whole teaspoon. There's 35 left! And if you use a good modern digital thermometer the probe is so thin there's no way you'll lose that much. So stop worrying about it.
Same thing for using a fork to turn the meat. Obviously tongs or spatulas are better for turning than a fork, but if you don't treat it like a voodoo doll, the juice loss will never be noticed.
The fastest way to lose moisture is to overcook the meat. Even 5°F can make a huge difference. For proper meat temps, read this guide.
Even slicing it to check doneness is not going to cause a major hemorrhage, although this is faaaar more traumatic than a thermometer or a fork turn or two. But the problem with using a knife is that the color you see can fool you. On a steak, for instance, many proteins contain iron ions, hemes, that change color when exposed to oxygen. So when you cut into that steak and think it looks perfect, after a few minutes it will turn redder. Worse, that light you have on your deck is not a perfectly color balanced white light. Incandescent lights are yellowish and looking at the color of meat bathed in a yellow glow can seriously fool you. The best way to tell if a steak is ready is with a good digital thermometer. Nothing else works.