You could theoretically write a script that would do it. However, you'd need to get the string and then have some kind of parser that would be able to identify each type of constraint and turn it into a mathematical expression or pass the string as a regular expression. You'd be reverse engineering the constraints and it would be pretty convoluted.

Hopefully somebody can put that in front of the Workflow team to look at. It would be nice to have some kind of activity that could take the results from a retrieve field values activity and spit back a list of valid/invalid fields. For some people it might be enough to use the constraints on the fields and template themselves. However, we tend to have very relaxed constraints up front and then enforce specific rules by having Workflow analyze the fields according to specific business rules. So, the ability to toggle between field constraints and manual constraints would be great.