Early D&D, for establishing the idea of character classes and having that as a defining feature, also has a skill system. It’s baked into the rules here and there without being made explicit; Lucy Blumire explains it well on her site, found here: https://rpghorizon.com/posts/2020-07-11_skill-system/

All of those skills can be compiled and serve as the base for an explicit skill system, of course. I think that may be a better approach, too, instead of dropping in a skill rating in the description of something else (“there’s a 2-in-6 chance of success”). The baked in approach drops in exceptions to the general chance in descriptions not associated with the description giving the general chance, in most cases, so learning the general rule and the exceptions is a bit trickier.

I’m leaning strongly to just making an explicit skill system to explain each of the abilities in detail and all can be found together. It’s still very much old school to have explicit skills–witness Traveller–and makes it apparent how many skills are included in the original fantasy systems even if they aren’t made clear.

That exercise also helps fill out and solidify skill lists. What does each class require? Of those skills, which do all characters possess in some degree? Everybody can sneak, for example–chance to surprise others–while some characters are much better at it–some by dint of birth and upbringing and other through intensive training.

I’ve found that also breaking things down by skills used in each pillar of play is helpful. What are all the magical abilities? Which are general and which only for specialists? Same with fighting and sneaking and speaking and so forth. Doing that makes it readily apparent how the different classes stand out from each other while also showing where non-specialists also have some ability.