i am not sure if there is an event but /eventtrace
Code:
https://wow.gamepedia.com/Eventtrace
tells you whenever a event fires
i just tested it and PLAYER_TALENT_UPDATE, PLAYER_PVP_TALENT_UPDATE and ACTIVE_TALENT_GROUP_CHANGED fired.
PLAYER_PVP_TALENT_UPDATE fired even tho i changed a normal (non pvp) talent