Forgeries are so common. It makes it difficult to sell things like this without accredited authentication.
In most cases I'd share your concern, but Stan will sign just about anything for a buck and he's at every major comic convention throughout the US year in and year out. So it's not too hard a leap to think that every Stan Lee signature out there is authentic.
If you were going to forge someone's signature, there are a lot more rare and lucrative ones in which to fake.