How can I know if my humble choice keys have region restriction?

If you press the gifting button, it will give you a pop-up that tells you.
Also check their respective Humble Choices posts here.

If you know the sub ID that the key redeems as you can check directly on steamdb.info... but you usually won't know that with a bundle key.

You best bet is probably to go look up the bundle you got it from in the "Deals" part of the forum. The folks managing those threads do a great job finding and sharing that info.

If you buy it from Humble Store, you can't
If it's from Humble Bundle, you can check the gift option

I faced this problem in the past https://www.steamgifts.com/discussion/OLAgz/humbe-bundle-store-region-lock

Now I switch to Fanatical for non-bundle key, they have nice region list feature.

As others said:

• If you click the gift button it will show you, you can than cancel the gifting by click the X on the region menu popup, and go back and redeem a key instead.
• Deals Forum, almost any bundle will have a dedicated thread. It may take a few hours to days for region information to be added though, as this isn't an automated thing, it relies on the feedback of users who purchase the bundle and report what their region has.

Warhammer - Yes 24 Regions
V Rising - Yes 37 Regions

Since the issue is region restrictions, you will have to ask the winner if it is okay to delete the giveaway, staff will not reroll if there are region restriction issues (creates an lengthy loop).

https://www.steamgifts.com/discussions/subscribed/search?sort=new&q=region 🤨
Not sure how you can't find anything about it in previous discussions. Could you explain your situation more then?

Also knowing the publisher pattern of region lock