Just by listening to it? or looking at it? Like he said ^^^ listen for a high pitched sound that gets louder the higher they rev, that's the turbo spooling up. After they let off the gas, listen for a *psh* sound (that's the blow off valve, if they have one). And of course, if you see inside the engine bay, look for a turbo, and on the outside, look in the grille area for an intercooler.