As Christopher said, it is all about training your eye. Start compiling exemplars for Mantle, and look at them every day. Study the fakes, study the inscriptions, pay attention to where he signed, what items were commonly signed when he was doing shows, how big or small he would sign certain items, etc. This is the fun part of the hobby to me!
There isn't one major tell tale sign, but most forgers can't even get the first "M" right. Some fail miserably in the "ic" shape, some fail miserably on the "an" connection, some on the "tle" shape, some on the baseline, signature placement,etc. But it has to be looked at as a whole.
Here's a bit of a "Starters Guide" I whipped up for you. Authentics/Atypical Authentics/Common Forgeries, and what I consider halfway decent forgery attempts that are slightly better than the normal crap you'll see.
