Hulls might be easy to find for you but I prefer to start with 3" then cut down to 2.5". The 3" are hard to find and I usually keep my own from waterfowl hunting and 2.5" are impossible to find here. I don't have any guns that are 2.75" chambered.
In that case, it's certainly simple enough to tell. If you can't see a basewad visually then take a small flat blade screwdriver and feel the interior to the bottom. It there's a bump a short distance from the bottom that's the edge of the basewad.
I bet you find very few that are one piece construction these days. Most will have a very thin straight wall tube and a cupped basewad. They will be best served by a wad intended for a straight wall hull.
