How would you know if a celebrity autograph is real or not?

So, I got an autographed picture of Joe Walsh in the mail today. Actually, its more of a postcard advertising for joewalsh.com. It is signed in silver sharpie, and from the address I used, I noticed that I was the first to post a success. I cannot tell if it is a pre-print, autopen, or if it is actually real. I'm not too good at telling yet, so can anyone help me out? Many thanks!