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.
For getting the decal off, as suggested above, but you may also check with a local body shop or a shop that specializes in wraps and wrap removal.
After you get the decals off, it may be a good time to do a thorough wash (by hand), clay and then either use an AIO such as HD Speed, or a mult-step process cut / polish & wax with a dual action machine. Remove all polishes and waxes by hand with soft plush microfiber.
This will a.) remove any swirls, RIDS and scratches that already exist and two, level out the areas where the decals/stripes were and you'll have a consistent looking glossy car.