Gift Card Database (GCDB) has a guide to spotting tampered gift cards:

Whilst it may seem unusual, you should tear open this version of Apple gift card before you purchase it so that you can inspect the redemption code. Look for missing or scratched off characters (it may be as subtle as changing an L to look like an I).

If you’re satisfied that the redemption code is legible and undamaged, you can purchase the gift card by scanning the barcode on the other side. If staff question your decision to open it first, calmly explain why you were checking it and refer them to the image above if it helps.

The one major downside of this precaution is that it requires you to basically destroy the gift card packaging so if it’s intended as a present you may just have to give them the smaller inner card instead. Still, it’s better to be safe than sorry.

I’m not bashful, but I’d be very uncomfortable opening gift cards before I purchased them. The whole point of this is that gift card scams are on the rise. If I saw someone opening gift cards in-store before purchasing them, I’d think they were shameless scammers. If you need to destroy the retail packaging for a gift card to feel certain it hasn’t been tampered with, the whole system seems fundamentally broken. (And just eyeballing the redemption code doesn’t prove it hasn’t been tampered with.)