This will sound very flippant, prof, but how do you know that you are eating chocolate or drinking coffee if there is no label on what is on your plate or in your cup? By what it looks like and what it tastes like. That is how I find out that Jim Mooney is drawing a particular episode of Lash Lightning. He has a style that is uniquely his at a particular time in his career and when you have seen enough of his work to learn how that style developed, then you can apply that knowledge to works that are not signed, just as you can drink some liquid and decide that it is coffee (or tea, or coke, or pepsi, or beer, or root beer, or dr. pepper, or whatever - they might all look quite similar on the surface but they will taste differently to you).
I don't believe that it is a "knack". It is more like an art. Or a gift. You either have the ability or the potential for developing it or you don't, but most people will never learn more than a little bit. I can barely play chopsticks on the piano. I will NEVER learn more than that. I can't. I don't know why, but I have no gift for music. I have the gift of recognizing artists. I don't know why I have that, either.
If you can develop the ability, more power to you. There are too few people with the "knack" today, and we need many more to carry on the research especially now that sites like DCM are making old comics more available to more people. Where are the new art spotters? I can only say that I treasure the friendship I've made with Tillmann and the fun we have going through each batch of comics for scanning twice a year. See you in a month, my friend.
Peace, Jim (|:{>