PostScript - Comparing values for equality + "if" structure

I have previously defined

/smth1 [0 1 0] def
/smth2 [-1 0 0] def

I need to check if they are equal, and if they are, perform some action...

For example, (Equal!) show.

I know that I should use eq and, possibly, something like

... {(Equal!) show} if 

But I can't figure out how to correctly compare the previously defined smth1 and smth2.

Please advise.