Laughingvergil

I am working on creating a mod to randomly grant an extra level occasionally in a related skill when a character gains a skill level. However, I haven't found out how to trigger an event when a player gains a level, or how to determine which skill has added a level. Eventually, I plant to break it down so that, if a level is gained in a Thief skill, then randomly an eligible thief skill (not at 100) will occasionally gain a level as well; the same for mage and fighter skills. The interesting side effect of this is that sometimes you will train things that you didn't focus on training. This is based on the idea that, sometimes, when you learn something, you also learn something in a related area. But first, I need to reliably detect when a skill has leveled, and what skill it was. Can anyone provide some guidance or hints for this? I'm willing to experiment, but am uncertain how to do this, or even if it can be done.