You can use this github website with the code of ESO to search fot the even name and check the code around it.
https://github.com/esoui/esoui/tree/master/esoui
Most of them are not documented and you are right about trial and error.
Add a callback function, let the function do something like d("Event <eventName> fired") and then you can go testing
I'd say EVENT_ACTION_SLOT_STATE_UPDATED is fired as the action slot's state changes (locked e.g. on weapon bar switch etc.) and the other is fired as the action bar slot chanegs in total (e.g. a skill get'S replaced). But these are only gusses.
If Is earch for EVENT_ACTION_SLOT_STATE_UPDATED in teh eso source code I find this file:
https://github.com/esoui/esoui/blob/...onbar.lua#L576
Reading the code tells me function HandleStateChanged is called and this function's code tells me the UpdateState function of a action bar button (determiend via function ZO_ActionBar_GetButton) is raised.
So you need to find the function ZO_ActionBar_GetButton to see what zo actin bar button is and what the function UpdateState does, and so on.
The action button:
https://github.com/esoui/esoui/blob/...tionbutton.lua
The UpdateState function:
https://github.com/esoui/esoui/blob/...utton.lua#L269
Seems the state change event checks if something is slotted on this action slot button and then changes the activated hightlight texture, and a status child-control