Edit: Sorry, I thought, the question was for ace: I just folowed the link from "recent topics" to this thread without checking.
Though I wonder if there isn't a similar method in XP.
Edit 2: I originally made my post thinking the question was about ACE and later edited it when I realized it was actually for XP. After doing some research (I.E- downloading the XP demo), I found out that everything I said still applies to XP, so here's my post again, with only a screenshoot change:
One of the options in the conditional branch menu allows you to check if an actor has learned a specific skill.
It's not really "easy"... but I'm not sure if there's another way. By using a common event, you can perform that check with every skill he can learn and activate a switch for each positive, that way you'll be able to "remeber" what skills your character knows.
Aside from using a script, I think that's the easiest way.