Hi, good questions all and asked at the right time - before the purchase.
OK - well, get to know the signature and what it should look like. And get to know what it should cost - photos are different then letters or checks etc. Check closed auction results like RR etc. Decide what you want, a signed photo, a letter etc. and get a feel for how they look. Authentication takes a long time to be skilled at, so you will likely and hopefully enlist the aid of a third party authentication service and or individuals. Sticking with established authentication companies, such as PSA/DNA, JSA or Becket is wise, but never assume anything is genuine just because some papers say so. This is where you ask opinions, perhaps from folks here. Also, just because something is authentic doesn't mean it is "nice" or desirable. It could be a poor example signature, or some odd item signed -who wants a signed photo of him eating a...Greek salad? Now, crushing a grapefruit...might be a bit different. And condition - watch condition. I'm sure I missed a lot - others will chime in to help.
Thanks....you put a smile on my face about the grapefruit. lol
I'm often just as pleased, or sometimes more so, with a signed album page.
I was wondering if there's a tell-tale sign like with Charleton Heston.