
Antariuk |

Supernatural abilities are magical but not spell-like. Supernatural abilities are not subject to spell resistance and do not function in areas where magic is suppressed or negated (such as an antimagic field). A supernatural ability's effect cannot be dispelled and is not subject to counterspells.
Since they are magical, you should be allowed by RAW to detect them with detect magic, arcane sight, or whatever. I once saw a (house)rule where someone allowed (Su) abilities to be identified with a proper Knowledge check tied to the creature in question, which I think makes a lot of sense, but other than that I'm not aware of solutions for this problem.

Claxon |

It would be reasonable to allow knowledge arcana or possibly spellcraft to identify an effect caused by a supernatural ability, especially since many of them funciton as spells. However, within the confines of RAW no such allowance exist and you would not be able to identify it.
Knowledge of a creature might allow you to know that it can use certain abilities and a general idea of how that ability functions.

Question |
Hmm i think detect magic doesnt allow you to figure out the spell or its properties though? Just the aura and strength.
Are there any options that allow you to identify spells in general, AFTER they have been cast? Because spellcraft seems to indicate you can only identify spells as they are being cast, but if you come across, say, a pre-existing wall of fire, you cant use spellcraft to figure out that its a wall of fire (excluding the common sense option).

Claxon |

Wow...so you could have a witch using hexes to kill people and nobody would have any way to catch her...
Most witch hexes wont outright kill someone on their own. Their has to be some other action to go with to kill an enemy normally.
Also, they have to be within 30ft to apply the hex. If someone is always within 30ft of a murder, you're going to make a connection.