A question on Skills / Skillchecking

[Jormaine]

when the opportunity comes to skillcheck (use of perception or reading a rune) arises, does the PC in question have to have a rank in said skill or have it as a class skill to even be eligible to do said skillcheck?

EX: there is a treasure chest in a small island, and it requires you to swim across. Does said PC have to have a rank in swim to be able to swim, or can they still attempt to skillcheck on swimming?

[Jorda75]

It depends on the skill. Trained only skills (Knowledge, Craft, ect) require at least 1 rank in the skill, regardless of whether or not it is a class skill (in Pathfinder class skills simply grant a +3 bonus as long as you have at least one rank in them).
Untrained skills (Perception, Stealth, Swim, ect) require no ranks and everyone who gets to make a skill check, adding the inappropriate ability modifier (and penalties for low ability scores and armor check penalties) to the roll.

[Jeraa]

Disable Device, Handle Animal, Knowledge*, Profession, Sleight of Hand, Spellcraft, and Use Magic Device are all Trained-only skills. You have to have at least 1 rank in those skills to attempt a skill check with them. Every other skill can be used by everyone - it doesn't matter if they have ranks in the skill or not.

*Knowledge skills can be used untrained, but only if the DC of the skill check is 10 or less. IF the DC is higher then 10, you must have a rank in the appropriate Knowledge skill to make a check.