How to know what variables are useable?

[AbsoluteVM]

I'm trying to find out an answer to what might seem trivial, but need some help.

How do you know what is referable by, for example $this->product->this_reference_here

Is there a list of variables that are made available, or a way of printing the contents and knowing what I can use.

[Milbo]

use in your layout vmdebug('my vars',$this); and enable debug in the config

[PRO]

There are category and product variables listed in the stickies here
http://forum.virtuemart.net/index.php?board=130.0