I have been playing since a while, and I am pretty sure that the only way you will land Critical Hits is by:
- some Feats guaranteeing Critical Hits, and
- increasing your Critical Hit Chance by Items, Base Abilities and Feats
That being said, there is quite plenty of options among those.
If somebody mentioned attacking from stealth, causing a Critical Hit, he might actually have meant the Snipe feat, which can only be done from stealth. (it is not technically a Critical though, but does bonus damage).
It could also have been confused with Ambush, which gives bonus Critical Chance based on your Stealth Skill. Does not require you to actually be in Stealth, but is dependant on lighting circumstances. With enough Stealth Skill, you can pretty much get guaranteed Criticals on every single hit. (I am currently playing a character based on this feat, and it CAN be powerful if done correctly - I might post the build.)
In terms of traps, they must have been referring to using the Deadly Snares feat, which makes all Crossbow Attacks against an enemy stuck in Bear Traps OR Acidic Entanglements into automatic Critical Hits.