The best way is to simply try - with a finger nail to grab an edge and peel - depending on the 'age' of the decal, once that is done adhesive remover and then some rubbing compound, there
'may' be a shadow of the decal on your paint - hard to describe, you can see it only in certain deflection of the sun, if its a nasty decal - light passes with a heat gun and or blow dryer to loosen the decal, but be careful w/the heat .....standard 8" to 6" away - invoke some patience, many passes, you can melt the paint.