how can i tell when i need to change the belts? Also, is it hard to change them?

When it (only one belt) starts skipping a lot and you hear a chirping noise when you brake, or the bumps wear down a lot, or they stretch so much you can't it.

The difficult part is putting the spur and assembly back in. Watch closely as you remove and if necessary draw a picture to remind yourself of how things fit together. You can also download the instructions from Losi website.