I think not necessary other guro fan will share your preferences.
There are no accessories or signs that guro fans wear or know.

so according to my conclusion, only way to identify potential friend is to put it into situation where you can observe emotional response towards guro content. (as you can accidentaly make someone observe guro pictures or some movie) and look how facial expression changes and what is the reaction.

or if you want to find out that in discussion you can try to talks about something abstract but related to guro and see where it goes.


but the problem is that all that is pretty hard to do and it would be preferable to find some method to identify potential friends from huge number of people.