Is there an easy way to check to see if a file is a valid DBF?

I'm issuing an APPEND FROM myfile.dbf. This works fine and dandy until someone points to a file that's not a DBF. VFP issues the error "Not a table." Is there a way to check for this before I issues to APPEND? For that matter, what do apps look at to determine what kind of file it's dealing with? Is there something reliable in the header?