When grilling burgers I usually flip several times, but have recently been advised to cook forever on once side, flip once, cook and be done.
Is that the best way to do it?
How do you know when a burger is ready to flip?
I'm solidly in the flip as often as you want camp. As long as it's not over (or under) cooked I find that you get better results flipping often. Plus you can rearrange things to make more room as you flip.
To tell how cooked a burger is poke it with a finger or something else that's not too sharp; just like a steak the more cooked it is the more firm it will be. You just need to learn how "squishy" you like them.
Also, as fun as it is, don't press the whole burger into the grill! All that happens is that the delicious juices are forced out.
Harald McGee says flipping more often will help the meat inside the burger (or steak) to warm without burning the outside.
Multiple flipping: more even cooking, thin crust Flip once: uneven cooking, thick crust
(Note: by 'uneven' I mean that you will have a much wider range of doneness from crust at the outside to possibly raw in the middle; more crust=less unevenness)
So it's a matter of taste really. I personally prefer multiple flipping, in a pan and not on the grill.
I'm in the flip-turn-flip camp to sear in the juices and for some sweet looking grill marks. FWIW, I used to be a high volume grill cook. After a while you can tell when it needs the flip or turn just by looking at it. It's also really weak to cut into the meat to check the temp since you'll lose all the juices.
I prefer "flip once". Assuming you've got the heat right...
I think the meat done-ness method I heard was to compare it to each of your "earlobe" (rare), "cheek", "side of nose", and "tip of nose" (well or medium well), going from least to most done. The more firm the meat is, the more done it is.
or you can do what my mother does, and get out a knife and cut into one of the burgers and see how pink it is on the inside.