For me, the litmus test is this: If an organization believes or claims that if you're not part of the solution, you're part of the problem, that organization is "fascist," i.e., will arrest you for minding your own damn business.

Of course, there are exceptions - war-time collaborators, say - which give color to all others making that claim, because every movement is fighting a "war" against an "occupying power." But wars are temporary and there is great consensus among the people that collaborators are bad; there is no "movement" to condemn them. Movements that demand that citizens snitch, shun, and scold those who don't actively join their ranks are dangerous in the same way.