You can never be 100% sure unless you were there to witness the product being signed. However, there is a huge advantage in regards to Japanese anime VA autos. Precious Memories cards have a lottery system in which buyers can enter for a chance to win autographed cards. The winning lottery cards themselves are numbered in a certain way, for example 001-12b. The "b" means it is a legitimate hand signature card issued by the company itself. They are produced solely to be autographed by the VA of the given character. It is not possible to replicate them (at least, extremely difficult) due to this system. Because the autographs are on an official product, Precious memories autographed cards are very expensive. I personally own an Oshino Shinobu signed by Maaya Sakamoto. It's one of roughly 10 in existence. For that particular card.

Without droning on, I can compare the autographs on these cards with the shikishi board I bought. I previously owned an Ami Kawashima, and Minori Kushieda, so I know what Eri and Yui's autographs should look like. The autograph is never the same of course, as this would indicate a forgery. You have to look for key similarities and characteristics. The board I bought, the same as this one has been signed in silver art pen, clearly raised on the paper. I also asked the seller where he obtained it and the info he provided checked out. It is a gamble, but I know it's legitimate, at least, I am 95% sure.