[mothered]

Yeah but I can't really know which of them are blocked, as far as I've researched

I understand.

I'd say It's based on both commonly used proxies and an IP range. The only way to get around this (that I can think of), Is to Identify through trial and error- which can be a very lengthy process. Of course, private proxies would most likely suffice but they come at a cost.

What do you mean by trial/error?  

Isolate those that do/don't work through Intensive testing.