Hi again,
For my collection, I've kept scans of (or own) authentic documents to compare signatures against. But that's not always possible. Also, I've seen reputable dealers truly believe in the provenance of their item but still be wrong. I recently had to tell one (VERY well-known) that a
Manfred von Richthofen signed card they were auctioning for $10,000+ was a fake. The signature was fairly accurate, but it was on one of Manfred's memorial cards (i.e., issued after his death). Like me, they didn't believe in signatures beyond the grave and removed it from their auction. I felt bad, but would have felt worse if someone had bought it.
Regards,
Lance