Joseph, the Gr”a talks about who the Erev Rav are and how to identify them. See Even Sheleima (in the new editions I think it’s perek 11. In the old ones it’s from siman 152 and onwards. http://www.hebrewbooks.org/pdfpager.aspx?req=49285&hilite=77bc56fa-eef6-4eee-9fc2-c8a336c15a15&st=%D7%A2%D7%A8%D7%91+%D7%A8%D7%91&pgnum=65)
To summarise he says there are five types:
Ba’alei machlokes and loshon hora
Ba’alei taavah
People whose insides don’t match their outsides (fakers)
Those who chase after kavod
Those who chase after money.