What values are available on events?

How can I find out what values are available with a given system event? For example, how can I find out what I can access and manipulate with OnLoadWebDocument?

Check page 697 of my book. The variables are listed in that section of the Appendix.

Ah. Didn't think of that - should have known you'd be way ahead of me Thanks.

Hm. On page 714 the Front-end system events are listed, but don't say what variables are available for them. Page 290 uses $modx->resource->_output with the OnWebPagePrerender event, but that is also in the example on the rtfm.

For almost all (maybe all) of the front-end events, no variables are sent in the invokeEvent() call. I think $modx->resource and $modx-user are always available there.