How can I check country restrictions on a key? Win Evil Within 2 if you can answer.

Never used Fanatical. But at least checked the restriction and it's so weird. You said a winner from India couldn't activate it but at the same time, if I pick India on Fanatical for Evil Within 2, it says that it will activate in India. So even the region restriction isn't accurate.

https://steamdb.info/app/601430/subs/ those are the existing subs (some with restrictions) for this game. My guess your key is this one https://steamdb.info/sub/209527/ or https://steamdb.info/sub/189512/, more probably the second one.

You can ask a friend who has this game to check the key SubID

Hmm weird, am Indian and it shows as being able to be activated in India.
Best bet is the steamdb check as Raziel pointed out.

You could cross check specific countries on Fanatical based on Steamdb sub info. In order to narrow down on the particular key Fanatical is selling you.
That is what I usually do, I check if keys are able to be activated in US and Germany. As I have the opposite problem in terms of regional restrictions. Since the key websites(eg: GMG) mostly restrict the nicer games to Asian countries in my case.

If you are in China then your key is for this package:
https://steamdb.info/sub/189512/

If you look through some of the old bundles this has been in there was a note here:
https://www.steamgifts.com/discussion/JIA8m/
in August 2022 that the key from Fanatical is region locked to China only, no other country, as seen in the SteamDB link.

Sadly that alert has been posted in some of the more recent bundles Evil Within 2 has been in.

There is no way to check a key's country restriction without risking to lose your key. The only way to check that is to activate it on a steam account.
You can first check all subs of a game on steamdb. If the steam account have all apps in all subs, you can say it is kind of "safe" to activate and check the subid of the key. But there is a chance that some subs is not indexed yet and some publishers have practice to add hidden app in bundled key. So there is always a risk of losing your key.
Other than activating the key on your/friend's account, submit to https://steamdb.info/keys/ is an other way to check the subid. But this has higher chance in losing your key.

I know that I bought some keys on amazon because they were super cheap and they were region locked to US and CA. I have read in various places that a lot of resellers do this region locking.

Steam doesn't have any such option. You can however ask steam support , providing them the key , and they might be able to help you. Alternatively, you can contact fanatical and ask them to give you a list of countries this key doesnt work on. Based on the data on their website, looks like its a ROW key, with restricitons on south east europe countries, some middle eastern and south america .. and most probably china as well since chinese region normally have their own regional key... good luck!