There really is no way to confirm (and unfortunately their sigs have been forged on a number of items). Other than plain experience in owning items you know for a fact are true sigs and just comparing them, its pretty difficult. That said, their sigs aren't THAT in demand so chances are, if something is signed I'd say 9 times out of 10 it's the real deal (the only time I might be particularly wary are on real old comics).
On another note, if you want to know for CERTAIN that you have a signed comic, seek out a Signature Series book. These books have authorized witnesses that are there when the book is signed in person. It is then encapsulated so it can no longer be tampered with. Here's an example on my Raphael book:
