Well, they're not really "hidden" exactly. Some values have built-in conditions to compare them, and some don't.
But okay, here's how you can find them:
If you can't find one of the expressions you need in a condition, you can use a compare condition. A "Pick by comparison" for the object will work, or you can use a System compare. (System compare won't pick the object though, so keep that in mind.)
<img src="http://i46.tinypic.com/20prjpv.png">
Okay, the parameters for the compare condition will come up now. Highlight the first field, and then double-click on the object for which you need the expression:
<img src="http://i46.tinypic.com/oh39ub.png">
A new window will pop up allowing you to choose the expression you need. If your object has a behavior or an effect on it there will be a tab at the top for those expressions.
<img src="http://i48.tinypic.com/161ydfq.png">
Double-click the one you want and it will populate the parameter you selected:
<img src="http://i45.tinypic.com/necbxt.png">
See, easy
You can also just type it in if you know the expression. You can also find a list of expressions for an object by right-clicking it in the layout and selecting "Object information." It will bring up this window:
<img src="http://i47.tinypic.com/2wqu23r.png">
The "Description" field shows the expression names rather than the name field, kind of backwards . But they're there. Also there should be a list of expressions for objects and behaviors and such on the wiki, but it's likely out of date. I think I may go and fill some in later while I have my coffee now that I think about it.
DAMMIT IT MADSTER I SPENT SO MUCH TIME ON THIS POST