@th Personally: "Dude. What the fuck is wrong with you?", and then in response to the likely "what do you mean?", start challenging them on why on earth they want to have anything to do with that, and drive the conversation with clear disapproval until it eventually ends up at pointing out that they're associating with fascists.

Basically: make the social disapproval abundantly clear, and give them *just* enough room to blurt out enough details that they can't retroactively claim they had no idea (which usually happens if you jump straight to the 'fascists' claim).

Applies more generally, too; be just hostile enough that someone feels challenged, but not so much that bystanders will dismiss you as the aggressor. The fascist will out themselves and their beliefs to onlookers soon enough, if you just keep them talking for long enough.