Scott, I am by no means an expert on signatures. I know there are companies that authenticate cards/balls/flats, etc that have been signed, but they are not foolproof. Search on JSA (James Spence) and PSA-DNA certified. While they add their professional opinions, I do not believe that opinion offers any kind of guarantee, which is problematic, in my opinion. That's why, for the most part, I only buy signatures on cards that have been certified by the manufacturer, ie Bowman Chrome prospect autos. These cards are signed in the presence of a company official to guarantee their authenticity. A finite number of the cards exist, and they are unique to the signed variety.
There is a real science behind authenticating signatures. If you are looking to buy a particular autograph, you'd be well served to find an exemplar for that person, and compare the auto you want to the exemplar. The problem is that over time, a person's signature can change. So, that's not foolproof. And though the exemplar might match the auto you're looking at, there are elements that a forensic analyst who specializes in signature authentication will see that you or I would not. They will consider physical evidence (the "flow" and angle of the signature, pen pressure, idiosyncrasies specific to the person's signature, etc), as well as provenance. That's real important when buying am historic signature-provenance. How did the seller come to own that signature? If you have somebody selling a signature of, say, Babe Ruth, and they have a sales receipt from a major auction, and that auction shows that the signature came from the Ruth family, that, then, would be pretty much air-tight provenance that the signature was legitimate.
My recommendation would be to take this question to the posters in our autograph forum.
Autograph forum
I would be very leery of buying signatures from players if I didn't witness them personally. I realize that there are many who are no longer living, and collecting their autographs would be a great addition to a collection. Just be very careful. I'm hoping the people who do this sort of thing on a regular basis will be able to give you some helpful advice on how to proceed.
Good luck!