How Do You Check Watermark When A Stamp Is On Cover?

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Morley Bright Rollatector (Roll-a-tector) Watermark detector and 2 new sealed Sachets (total 3 sachets). Detect watermarks on stamps or covers. From Company Web Site: Using the same principle as our best selling watermark detector, the Inst-a-tector, this Morley Bright Roll-a-tector, uses a roller to reveal the watermark. The image of the watermark remains on the ink sachet until another stamp is tested therefore giving to viewer more time to establish the watermark. Can be used to look at watermarks on stamps which are still on cover. The Morley Bright system uses the fractional difference between watermarked areas of the stamp and the remaining area to show relief when the ink in the sealed sachet is dispersed by pressure leaving 'tell tale' watermark traces. Place stamp face down between the special 'ink'sachet then gently roll the stainless steel roller over the top of the sachet, making sure you apply an even pressure across the roller handle to reveal the watermark Stamps can also be tested when still on covers. If the first 'roll' does not reveal enough of the watermark, make sure the stamp is in the same position and roll again - the final result will remain in the sachet until you use the detector again, making it ideal for cross checking watermarks with your stamp catalogue.. When the unit is 'rolled' the special 'ink' within the sachet will flow into the watermarked section of the stamp, allowing you to clearly see the design of the watermark, whether its inverted, the position of crowns and scripts and other important watermark features. Can also be used on recess printed stamps, simply place the extra foil supplied with the detector underneath the stamp this helps to soak up the ridges on the stamp aiding detection. Watermark detector is supplied with a protective case to store the detector and instructions in English, and also a watermarked stamp which we would suggest you test when first using the detector to help you appreciate what you should be looking for.. No fluid comes into contact with the stamp, no batteries are required and the Roll-a-Tector fits neatly into your pocket.

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It is generally impossible to check for a watermark still on cover.
One of the things you should consider first, before you do anything. Is this stamp really worth lifting from the cover?

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Here's the link to when I asked the same question. I added the result of the scan. And the stamp is still on the cover!