Is there a way to know if the autograph is authentic with 100% certainty? No. Not through the mail. The only way to do that is to receive the signature in person. However, collecting through the mail is a fun hobby and just takes some faith. A lot of the time you can tell if the signature is a sec or a pre-print just based on what others have posted online and doing a comparison. Pre-prints are usually easy enough to spot when you shine in the light and see that the signature is a part of the actual photo. End result? Just have fun with it! Pre-print, sec or authentic - someone took the time to send you SOMETHING and that's pretty cool in and of itself.

Is there any way we can know for sure that the autographs we get are real? Some celebs have agents who just send out pre signed pictures.