1 Be sceptical of COAs - COAs do NOT provide a guarantee of authenticity. To the contrary they are used by unscrupulous sellers to lure the buyer into a false sense of security.
2 Check Dates - Ask to confirm the publishing date on the reverse/cover of all publications, LP, Singles, EPs and the like
3 Do your own Research - Don't buy until you have satisfied yourself that the key authentication criteria exaplained above have been met. Review authenticated samples and known "Autopens" examples.
4 Consult with Experts - If necessary consult with recognised experts in the field.
5 Buy from reputable and accredited Autograph Dealers - Look for UACC / PADA / AFTAL Dealer Credentials
6 Seek a "Lifetime Moneyback Guarantee" - Reputable dealers will be happy to provide a moneyback guarentee without time limit if an autograph is subsequently found to be fake.
7 Contracts/Personal Checks/Handwritten Documents are harder to forge - Forgers tend to confine themselves to small "cut signatures" or adding their scribble on top of photographs, books, magazines etc. Larger documents, perhaps containing official stamps, or check cancellation marks, additional handwriting or annotations are intrinsically much harder to forge.
8 eBay: Check Seller Feedback & Ratings - Check for too many appearances of "rare" items; look for price anomalies versus the wider market (for investment grade items particularly).
9 eBay: Be wary of inaccurate item descriptions - A description could contain genuine mistakes, or more sinister inaccuracies; which immediately raise doubt as to any authentication claims
10 Trust your own instincts - If something sounds too good to be true, or it it just doesn't feel right - then trust your own judgement.