I believe the way to tell wfs ones from other similar ashtrays is that in wfs ones, if can hook your finger under the rim a little bit, its wfs. The dimensions are not absolutely consistent in these, I have one a full half inch taller than another, and the shape starts to become more ball-ish - curving in a bit. But there is still a ridge under the rim of the short one.
The bubbles should be well-controlled and the colour should fit in with one of the "designated" wfs colours.
(mine are both full of fag ends!)