[belk07]

I have a real simple question about resonance that apparently points out a huge gap in my knowledge. 

If one is given a molecular formula, how does one tell that it has a resonance description? In other words, if I am given a molecular formula, how can I tell that I have to draw multiple Lewis structures or that one will suffice?

The text book just gives examples of molecules with resonance but does not tell how to determine that resonance exists. I'm fearing that on a test I'm going to be asked to draw Lewis structures of various molecules and will miss those that need a resonance description.

Thanks in advance.

[grs35]

you can usually guess it from the molecular geometry of the compound.