A couple of ways to check without screwing with the paint or finish are to pull the fork and bb and clean stuff up if it is indeed full chrome you should likely be able to see some chrome under the paint. This will also allow you an opportunity to service the bearings and inspect the inside of the frame for issues like rust, since the bike likely needs service inspection this kills two birds with one stone. Also if you send us some good pics of the bike someone here will likely know if it's full chrome under or just silver colored under coat with colored over which was common on 60's and 70's bikes.