Something like

   /\\ref\{\s*\}/

in the Pattern section of the preferences ought to be enough. The  
double backslash gives a "real" backslash (it's usually used to  
escape other characters, such as the "\s*" to denote any amount of  
space between the braces above, or conversely, again as above, to  
indicate a *literal* brace). Having the "\s*" inside the braces means  
that if you accidentally leave blank space inside it'll still be  
flagged.

Cheers,
Paul