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.)
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:
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.
Double-click the one you want and it will populate the parameter you selected:
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:
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 Also there should be a list of expressions for objects and behaviors and such on the wiki, but it's likely out of date.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