Cooking juicy beef in a pan

The best way to test if it's done is to use an instant-read thermometer. Slide it into the thickest part, away from any bones. I like to lift up the meat with tongs and poke it into the edge, all the way to the center. You're looking for about 135-140F for medium rare; take it off a little early if anything because the temperature will go up a bit once it's off the heat. Let it rest for ten minutes or so before digging in to let the juices re-distribute.

I don't like using nonstick for steak, or olive oil. Neither does well at high heats, which is what you need to cook off moisture at the surface and get a nice browned outside before the inside gets overcooked. Cast iron and steel (stainless or carbon) both work very well in my experience. For fat, use a high smoke point oil like safflower oil, or clarified butter.

In addition to what Adam said, I would also include the quality of the meat. Most of the meat you can get in your average megamarket is going to be tough and not the greatest choice when you want to cook hot, fast and under medium. Finding a good butcher and spending that extra buck per pound, will help ensure a tender chunk of beast. My favorite way to cook a steak is the Alton Brown method. Fire up my charcoal chimney starter, salt the steak and place the chimney on top of the steak for a minute and a half. Flip it over, another minute and let the steak rest. Best steak I've ever had (and made).

When you're grilling or pan frying, toughness with beef is correlated directly to cooking time. The more you cook it, the tougher it's going to be. In fact, probably the easiest way to check for doneness, without use of a thermometer, is by pressing on the meat. If it's soft, it's raw. If it's hard, it's well done.

Now, for expensive cuts like fillet, you have some leeway. Fillet is a very tender cut, and it can handle a lot more cooking and still be south of chewy. On the other hand, a tough cut like flank steak or skirt steak will tolerate very little heat before it becomes hard as a car tire (though here, you can cut across the grain of the mean to somewhat mitigate this).

As for safety, generally a quick sear on the outside kills everything you really need to worry about. With steak, your main worry is contamination from handling and processing, and that's nearly always external. So sear the outside, and your steak is safer than the side salad you're having with it.

My experience with beef is that if you want good results with cheap parts you should cut in very thin slices and cook quickly keeping pan very hot. If you like thick "saignant" it should be very good quality meat and you should rub it with fine salt and the ground spices you like, then burn in very hot pan for about 30 seconds every side until you cook it externally to keep all juices inside. When the color is right, lower the heat source to avoid charring and continue cooking as you like. Remember that when you see juices coming out from meat it should be retired from heat because will become soon dehydrated and hard to chew. I'm obtaining very good results marinating hard meats with milk, yogurt and spices for one or two weeks at about 2°C covered in fridge. Will try to add some pineapple and ginger slices to shorten times. When marinating to tenderize don't add salt but keep liquid acidic to avoid poisonous bacterial growth.