I actually wanna buy Bioshock Infinite from Steamtrades, but am scared of getting scammed. Don't want a Russian version obviously. How do I know if it's a legit English version of the game? Is there any proof that I can get, or how is this done?
No, you cannot test or check a Key otherwise. You are totally bound to the word of the seller. What you could do is ask for the recipe EMail that they got when buying the game. I personally would refuse it if it contains my payment data.
The best way is just to trade with people that have some positive reputation on Steamtrades, but I also never trade keys, only Steamgifts.
In Terms of a Steamgift (Item from the inventory), if there are different versions, they often contain abbreviations to show what version they are.
Some common abbreviations:
So if you get a Steam Gift in a trade that contains RU or LV or GER/DE, you might wanna reject it.
There's no way to check a key without activating it, but you can demand the key up-front - it is a common practice when trading inventory items for keys. Tell them you aren't giving them anything if the key isn't for an international version or if it doesn't work. Obviously, you are very unlikely to get a bad key that way, since nobody is going to give you a region-locked key if they know they aren't getting anything for it. Some people may reject that offer, but there may be others, and having a high reputation on steamtrades helps convince people you aren't scamming them.