Base
Heyne always used a dark red/bown clay. Items typically have some form of identification marking on the base. Base markings are hand inscribed and generally very difficult to discern.

Some items are more clearly marked “Hey”. Sometimes it appears to read more like Hoy (left image below), hence the previous miss-attribution to a fictitious company called Hoy.

Stickers/Labels
Very rarely were stickers used. I’ve only ever seen a photo of one item with a Heyne company sticker.