Watermark Fluid...opinions Wanted

What really mystifies me is when sellers advertise covers for sale, specifying the watermark of the stamp/s on the cover.

In some cases, the stamps could be one of two or even three issues with different watermarks.

How can they do this?

So far, I've only experienced one case of a cover where the envelope paper was so thin that you could establish the watermark of the stamps by looking through from the paper at the back.

Mhc99, yes I do deserve that question. See back then when learned senior collectors knew I had good eyes the advice given to detect w/m's was; a flat black backing, angled light and a quick dip in distilled water (if necessary).

After practicing the w/m did show up, although it did help knowing the shape/style of w/m one was looking for. I found some New Zealand took patience.
Small diagram showing what I mean by no glare 'Angled light'.

Good on you finches!
Water as a watermark detector never seems to be mentioned.
It comes in very handy when you have a lot of very dirty KGVs
in a bulk lots that have more gum on them than the day they
were printed and totally covered in dust to top it off.
These are the KGV buys I like the most. As they dry 99.9%
show you their watermark with surprising ease. It is the
quickest way to do a lot of watermark checks in one go and
it is so easy to do.

I must admit that I am talking about cleaning stamps
and not a quick dip in water.

Finches, thanks for explaining how to use the flat black surface, water and angled light on used stamps but not on mint.
Regards
Mike

KGV Collector, yes that's a rarely mentioned point, the drying phase and how the w/m becomes very evident at a certain stage of drying. Maybe collector do other tasks while waiting for stamps to dry.