It all depends on the marble. If it's a total mystery, then I look at the pattern and swirl construction. That usually trumps all, but not always. Some ravenswoods and Alleys are nearly identical and it's just hard or impossible to tell the difference. Some marbles are very recognizable for their glass, like the champion furnace marbles I posted, or many Akro marbles. No matter what the marble looks like, you can tell by the glass what it is. Others are recognizable by their patterns, like peltiers. No matter the color, a pelt swirl is usually very recognizable by the pattern alone.
I have quite a few swirls sitting in a box, if I took a collective pic of some, would you be able to pick out any that I could pull for further ID? Or would you prefer me to post like I did above. I just don't want to bore you with my stuff as none of it is high end!
I don't mind at all, I enjoy it. Usually I check these forums while I'm watching TV in the evening and IDing marbles keeps me in practice! I don't always get every one right, but I'm decent enough at it.