What exactly you need to verify? Soul Reaver 1? Original GOG release or modern uldate that rid happen here on GOG and removed reverted? Or Retail CD release?

Verify first file names of exe/bin. During setup make sure you check "Verify integrity" checkbox. This for bin files verifications. As for main exe file - there should be Digital Signature tab, when Windows should report "Sign valid" at additional info tab (+date, company GOG SP zoo, email etc).

There is some ways to verify disc images too.

I wonder when this is going to be available for purchase again. I own the whole series on GOG except this one. I completed Blood Omen last year and haven´t played the others because I want to play them in order.

Maximum you can try to get it legally - GOG or Steam keys for game or series bundle. Or lurk file name mentioned above. In any way better lurk for that modern setup, even as a base for fan-made fixes (old build of GOG have converted video and intro misplaced by pre-release promo/trailer version, or something like that). Modern build is just clear original game version with GOGs DX Wrapper (which should be renamed if you want use Wrace's fix mod wrapper).