I'd suggest looking at the source code for the default UI. There are a few online repositories that have this available or you could
extract it yourself. It would be a good idea to list all the different ways the game can process mounting to get an idea of what you need to look for. Things like ActionButton, Spell/Item use, CollectionFrame, etc.
This list may get overwhelmingly long, so you may want to consider if your code can support a more focused approach. For example, all these methods send a spellcast request to the server. The way to detect these is to listen for the server to respond with a spellcast start event with the SpellID matching the mount that's being summoned.