How do i know, if a key is region locked?

If its region locked it should be stated in product details, like in the dead island goty case: http://www.amazon.com/Dead-Island-Game-Edition-Online/dp/B008NAYASM/ref=sr_1_1?ie=UTF8&qid=1370440060&sr=8-1&keywords=dead+island
I checked it and it is available only in USA.

If you're activating it yourself, just activate it.
If its region locked it won't work, if not then it should be added to your library.

I have activated darkness II from amazon.com in Italy,don't know if that helps tough

I've bought a Darkness 2 key from Amazon and activated it without problems.

Das Spiel ist ungeschnitten, wie man an meinen Screenshots sehen kann.

by the way: search function

Ich bin aus der Türkei und Ich kaufte The Darkness 2 von Amazon und hatte kein problem.

I think its region free :)