As far as I'm aware, the only way to know if a Steam key works no matter how old it is, is to try to redeem it. Having said that, I have Steam keys that are over 2 years old and I have recently redeemed some with no problems. I have never encountered a case in which I bought a Steam key either from a legitimate storefront nor a bundle in which the key later expired before I redeemed it, and I'm unaware of any incidence in which that has ever happened for anyone other than perhaps time-limited promotion codes for freebie games or similar such as the extra bonus keys that came along with Insurgency in a recent bundle, where you got Insurgency with the bundle with no time limit on it, but it magically came with 3 extra bonus keys for your friends that did have an expiry date on them.
I should add to this; I have come across keys that have not worked that I have definitely not used, and after contacting support for the bundle site, they have emailed me keys. Most major bundle sites have very good support. Speaking of which, IndieGala has failed to get back to me about not getting an extra happy hour bundle, so I'll have to hound them again. In the past they have given me an extra bundle or two when they have stuffed up. In this case, it might be my error as I may have had the page open for ages before purchasing the bundle.
pigdog: Sniper Elite Nazi Zombies 2
£2.50 on GG. Steam only
Or US$3.75 (3750 blue coins) on the US GG site