Hate to tell you this but their is no easy way to tell , like 565 bbchevy said most people use aftermarket stroker kits . I built one back a couple years ago and used a scat system its just easier comes complete and balanced , ready to go . If you really want to confirm if its a 383 or not you can pull a plug a measure cylinder volume with piston at BDC . This is assuming the guy selling you the car understands what your doing ! If he doesn't he is not going to like the idea of you putting fluid down the cylinder and then retrieving to measure , and this is a B CH to do in car especially with headers !