Depending on the exact bow you have, there are things that can go "out of tune". Cam synchronization for example. You may or may not see signs of this by looking at arrow flight alone.

Single cam bows are supposed to be better at staying "in tune", but I've read they can need a tune up once in a while. I don't have one though. My bow is an old Hoyt with wheels, andI tune it by making sure the wheelsroll over at the same time, by comparing the positions of bushings inboth wheels relative to the limbs. I adjust it by twisting (shortening) or untwisting (lengthening) the idle strings. Once the wheels are timed, there istiller to think about. Mine is supposed to be tillered to about 1/16" or 1/8" less on the lower limb than the top limb. You adjust that with the limb bolts. Then it boils down to arrows, rest, plunger, sights, etc.