Decent Watermark Detector For Covers?

I am frustrated by the relatively large number of covers in my collection that I am forced to label as Scott "A" or Scott "B" because I can't discern the watermark. I do not consider lifting of stamps to be an option for my covers. While I know that nothing would be 100% reliable for this purpose without lifting stamps, has anyone had good experience with any device, either for watermarks or discerning faint grills, damage, etc while on cover? Are there any electric watermark detectors that accomodate covers?

Jax, I have the same problem. Does anyone know a good watermark detector for using with stamps on piece or cover?

Morley Bright Instatectors come in two forms, one for stamps off-paper, one for those on cover. I must confess I have enjoyed little or no success in using either, but I presume that reflects my inability, rather than their design.

Geoff

Thx Geoff. I rely on lighter fluid and my old watermark tray (not sure you can buy them these days?) for off-paper watermarks. I'll have a look at the Instatectors next time I am back in the UK. Agree that ability (age / eyesight) and patience is a key with watermarks as they are not always as clear as they should be :-/