Class skills are determined by the class, not by the skill or even the hero. So, you'll need to look at all the classes to determine if it's a class skill.
Do you want it to be a class skill for a specific class or for any class? For any class, you might try something like this:
Code:
var classskill as number
foreach pick in hero from BaseClHelp where "BaseSkill.nen2"
classskill += 1
nexteach
doneif (classskill = 0)
I assumed "nen2" is the thingid of your skill based on the error message. If not, put the thingid of the skill in question in its place.