I tried with a laser pointer from an old airgun and for sure the reflected beams of the two sides of the filter are different but not in the way they told you. Although my lenses are meniscus, not planoconvex, so they have a convex and a concave side (instead of flat) but the reflected beam from the convex side are several short lines and from the concave a single longer line. Maybe I'm not doing it right. The shapes of the reflected beams change a lot with a minimal variation of the angle of incidence of the laser on the filter.
Anyway, with meniscus filters is very easy to tell apart the convex and the concave side.
On a side note, I've just lost an auction on eBay for a ZM25. It was a half hearted bet from my part, as I'm not really convinced until reading Guy's repport on the performance of this lens with the 1m filter.
Nevertheless, the final price was lower than I expected for a copy in apparently great shape : 463 euro i.e. less than $500.
