How do you know you need to replace a rotor?

If you drove your car for too long and the pads were worn out then there will be small grooves or either big grooves from were the metal was rubbing against the rotor. If it has big grooves in it then they probably need to be changed or on some you can turn your rotor around and still use it. If it has small grooves it should be ok. Hope this is some help to you.