check for smoke from the exhaust black is fuel white is water blue is oil, take of the oil filler cap when the engine is ticking over and look for any smokey fume coming out as this is back pressure, ask if they have proof of the belt change, try and drive it with the hand brake on to check if the clutch is slipping, press down heavy on each corner and watch to see how the car returns if its bouncy theirs a prob with the suspension, look from the rear along each side of the car to see if there are any areas that look like they have or had damage, check tyres for tread and uneven ware, just so