Detect empty parameters in ToolValidator?

[JamesHook]

This may be a dumb question, but how do you determine in the ToolValidator class whether an input parameter has been left blank?

Some of the parameters in my tool are optional in some cases, and required in others.  I would like to raise an error when a field is left blank under certain circumstances.

[LoganPugh]

if not self.params[0].value:
   <add some error message>

[JamesHook]

Thanks for your help!