This is a list of the available Joomla events:
As I don't know what exactly you're trying to do you might just want to browse the list and see if anything sounds suitable to you. You could then just use the event handler in your plug in. If you need any explanation as to how to use these events just let me know
Edit:just noticed that you were asking for a js event to handle your fields. You can do it in php with the joomla events too. The handler will give you access to the fields - returning false should abort the operation. Is this what you need?