You can look for re-welded axle tubes and spring perches on the rear end. Also Motion normally mounted dual fuel pumps and a C/D ignition as well. There just isn't a good way to tell if it is legit.
You must do a thorough ownership search, talk to prior owners and then if all looks good, contact Joel Rosen. It might cost you $2000 or so to get an authenticity letter from him, but without real documentation it's merely a disassembled hot rodded non-original BB camaro and should be priced accordingly. If the seller is so sure it's legit, have him front Joel's fee.
One other thing, for it to have real value as a BM car, it must have been modified and sold as new through Baldwin Chev. not a used car that was hot rodded by Motion later in its life.
Let us know how it turns out.