Pads wear first
Pads are designed to wear faster than rotors. Replacing pads alone is very common when the rotors are still in good shape.
When rotors should follow
A vibration in the pedal or steering wheel when braking can come from a warped rotor or uneven thickness.
Deep grooves, a pronounced lip on the outer edge, heavy rust or bluish patches (overheating) are also signs of wear.
Each rotor carries its minimum allowed thickness (often stamped on its side or hub). Below that value it must be replaced, even if it looks fine.
Why replace them together
New pads on a worn or uneven rotor wear badly and make noise. When the rotor is near its limit, replace both at the same time, by axle.
Rotor with hub
Some vehicles use a rotor and hub in one piece. In that case choose the part by the vehicle's year, make and model.
This advice is general. If in doubt, have your brakes inspected by a mechanic before driving.