Sure, you can use a meat thermometer, but you're waaaay cooler than that. Besides, a thermometer pokes a hole in that great sear you worked so hard for on that Porterhouse, letting all the juices run into your grill... where nobody can enjoy them (except maybe Vulcan.)
Watch the pros:
It's all done by feel. They know by touch how done the meat is. Cool huh? But, how do they know? Well, experience. They cook 100's of lbs of beef each season, and that calibrates that pro-quality, meat poking digit to know Rare from Medium Rare.
Here's a little trick to get a jump start on that experience. Calibrate your finger with your own personal steak done-ness meter. You already own one.. well, two actually. The fleshy part of your palm right below your thumb is an amazing analog (that's right, analog, look it up) for raw beef.
Lay your relaxed hand in you lap palm up, and press with the opposite index finger. That's what raw steak feels like.
Touch your index finger to your thumb in a circle (like saying OH-Tay!)
Now Middle finger to thumb
And so on and so forth...
It's not as accurate as a meat thermometer, but you won't loose all those delicious juices to the fire gods...
And you'll impress you party guests.
Now, get your grill on...