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.
cooking.stackexchange.com/users/2094/Eric Goodwin
Harald McGee says flipping more often will help the meat inside the burger (or steak) to warm without burning the outside.
cooking.stackexchange.com/users/641/BaffledCook
Multiple flipping: more even cooking, thin crustFlip 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.
cooking.stackexchange.com/users/None/
i feel flipping any thing on the grill once for best results juices stay in side place the burger or steak on the grill few minutes or so depending on temp
turn it
weight 3-5 minutes then flip it once better diamonds juicier steak or burger is
cooking.stackexchange.com/users/7307/scott
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.
cooking.stackexchange.com/users/1593/dotjoe
I prefer "flip once".
Assuming you've got the heat right... put them on the grill or pan as soon as they're all on the heat, give each one a little nudge to make sure it's not stuck finish messing with condiments or have a beer or something else for 3-5 minutes flip the burgers, and give them the same "unsticking" nudge as before and wait another 3-5 minutes get the buns toasted serve and eat
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.
cooking.stackexchange.com/users/4535/baka
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?
Grilling burgers: flip once, or keep flipping?
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<grilling><hamburgers>
cooking.stackexchange.com/questions/6851
cooking.stackexchange.com/users/2299/Jay
Flipping the burgers may give more even cooking without burning on the outside, but you may lose some of the juices. When flipping once, you can nudge the burgers after placing them to make sure that they are not stuck and leave them for 3-5 minutes before flipping, nudging again and leaving for the same amount of time. You can flip the burger after 3-5 minutes but you can also flip it multple times until it's done on both sides.
You can flip as many times as you want. The burger will cook more evenly. Flip only once and wait 3-5 minutes for each side.