To me, alleys have traits that tend to go with the factory they came from. There are ones in grey areas, but in general if you put 1000 sistersville alleys in a pile, 1000 pennsboros in a pile, and 1000 St. Marys in a pile, it would be very easy to identify which pile came from where, although not every single marble will be easy to tell by itself. Some of them are though. Like the Alley random group Winnie posted... the majority of those are Pennsboros, a few Sistersvilles, and one St. Marys.
I might be stepping out on a limb but if you never step out on a limb, you never make it to the next tree!