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.