does anyone know what symbology house dres uses to identify themselves.

None that I am aware of, other than slave driving.

EDIT: I'd say use pages 16 - 20 on the http://imperial-library.info/content/page-14-0 as the closest thing that resembles Dres as far as I am aware.

Usually http://www.imperial-library.info/content/great-house-dresis pretty good, but with TIL's downtime it doesn't seem to have come back up yet. Unlike the other Houses we don't get to see their flag in game, so we don't have their "seal" or anything. That I know of, at least; sometimes there are some obscure places that mention these kinds of things :shrug: Do a forum search maybe?

Yea, we're still in the process of moving FSG articles. That specific one wasn't particularly accurate anyway, iirc. As far as I know, there is no depiction of the Dres sigil.