You can't easily verify this, unless you find apparent data manipulation issues within the wallet.dat file itself or find the actual wallet encryption passphrase (very unlikely to happen).

I can't tell with certainty, but most circulating wallet.dat files for sale are fabricated and not worth any amount to pay for them. You're very very likely wasting your time with any of them. If you paid for them, it's very very likely a more or less expensive lesson.