Quick Note: Almost every roll can be argued except for a few. Wit + Com the general "Awareness" check and you can use it for most basic "spotting" tasks. It's one of the relatively few "Attribute tasks" specified in the core. For noticing other things usually Wit + Skill works well (e.g. Wit + Survival to notice a wild animal, Wit + Investigation to notice a lead other people have missed, Wit + Empathy to perceive a target's current emotional state, etc.) Basically, if you've got no real reason for a character to have skills related to the perception (just general awareness) go Wit + Com. If a skill would make sense in assisting finding some information, roll Wit + Skill (for reacting quickly) or something like Intelligence + Skill (For more methodical notice). There is no ironclad use of particular skills for rolls - if it would make sense to use a particular skill, use it. Most skills have a degree of overlap but will give the players different information on success.