andy has a great point about the variables.
but, variables aside:
i'm goin' with the chief once again. besides learning the particulars of your grill like the back of your hand (umm, ouch. maybe that's a bad expression for grilling
), i learned how to check if chicken breasts or tenderloins are done while working the kitchen at our baseball field's kitchen from the head cook.
we have to crank out food so fast from a tiny it'll make your head spin, so sending out under-cooked chicken was our greatest concern.
once you've grilled the boneless chicken on each side for a few minutes, pick it up in a pair of tongs and try to bend it until it cracks a bit.
if it just bends, it isn't done. if it cracks just a little and there's no pink inside, it's done. if no juices run when cracked, you overcooked it. anywhere from just a little to shoe leather. but if you've got it right, it'll be delicious.
if you do this a number of times you'll get to know your grill, the approximate heat, and the time needed to cook various thicknesses.