The best way to inspect a used treadmill is to actually use it. Try it at different speeds to get a feel for the treadmill belt and make sure it’s not shaky or skipping. Pay attention to how the motor sounds, too. You don’t want to hear any weird grinding or screeching noises, especially as you reach higher speeds. Make sure you’re doing a visual inspection too. Check the belt for signs of wear or fraying, as belt replacement can be costly.