which variables in which hook?

[rmd1]

hey!

since I couldnt find an mod that does what I want, and I cant get another coder on boat right now, I decided to code a plugin myself...

I started by finding out which hooks I need to work with, but already came across a problem..
where/how can I find out, which variables are already set, inside hook XYZ?

f.e. right now I need to run a query when a user writes a new post. so created a plugin which kicks with the usage of hook "postdata_start". now I want to save the "userid" of the posting user, but all vars I tried out are empty.
$userinfo['userid']
$this->userinfo['userid']
$vbulletin->userinfo['userid']

Non of them work. Can somebody point me in the right direction? Maybe not just to get the userid, but how to find out which vars are available at any hook.

Thanks in Advance !