A bike needs a tune-up if:

(1) it doesn't shift properly, ie. one click on the shifter doesn't always result in a gear shift, or the chain slips or jumps between gears without shifting, or you can't reach one or more of the gears, or you drop the chain off the front chainrings frequently, or it just runs very noisily (indicating the chain is not centered on a sprocket).

(2) the brakes levers almost bottom out on the bars when braking hard, or the lever position with the brake on doesn't suit you.

(3) the wheels are out of true, ie. the rim wobbles side to side more than 2 mm or touches the brake pads when the brakes are not engaged.

(4) unusual noises or play anywhere it shouldn't be