Identifying petrol stations as full/self

I'm looking for a reliable way to identify petrol stations as being full service, or self service. I do prefer one over the other, but the important thing is, I guess, not to needlessly get out of the car, or needlessly stay sat. I just want to "do it right" :)

I know about the signs フル and セルフ, but what if neither is displayed anywhere?
And also I think the moniker "SS" in their names just means "service station"? Please correct me if I'm wrong.

What other methods of identifying one over the other can be used?

I can imagine maybe certain brands, or locations such as along expressways, are always one or the other? I dunno, I'm just guessing. Mind you, I'm not looking to identify one specific petrol station, but any petrol station in general.