"How do you tell the difference?" Aside from common sense, one could...
Start by cracking open a few books. 
One could start with the better known contemporary texts, 'Sociology of Religion' by Weber and a good follow-up text, Comparative Studies in Sociology by Cousineau. If feeling especially daring and clever by half, one may even try the classics, from 'First Principles of the Reformation by Martin Luther to relevant Greek and Hellenistic annotated primary source material.
On a bad day, when feeling lazy or mentally spent, even a common dictionary allows one the critical and relevant differences.
But I would imagine that an intractable dogma which denies any relevant difference between cult or religion will be rigidly adhered to-- if for no other reason than it's simply more convenient that way...
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