Can you provide details and I'll attempt
Paul, it is referred to as "Watermarking"
The general option, is to purchase a tin of "Ronsonal" (Cigarette lighter fluid)
from a tobacconist, or suitable deli.
(I have trouble, as being a flammable
liquid, it is now banned from
deli shops in Western Australia)
To use, drop a few drops of the lighter fluid, on a dark glass plate or similar,
and pop the stamp in face down.
The watermark should be immediately recognisable, if it is there.
Lift the stamp out, and it dries without harm in about 2 minutes.
Needless to say FLAMMABLE requires suitable care in its use.
Keep well away from flame
Here is a dealer, watermarking a possible $1,000,000 stamp
using that technique. (The stamp turned out to be $50)