Fugitive Ink - How Do Check Watermark?

I'm searching through some postage dues and see many of them that have obviously been put in fluid to soak off cover or check watermarks. How would you check a watermark on fugitive ink stamps. I've tried lighter fluid before and it can effect the ink too. Is it a matter of how long it is in the fluid? Any tips? Thanks.

Oblique lightning will sometimes help. So will often will backlighting. There are devices out that don't use fluid, but I have no experience with them and hope that others will answer you.

I've used watermark fluid (Ronsonol) for years, and never, ever, saw any damage or indication that it was used to check for watermarks on any stamps, fugitive or non-fugitive inks. That fluid cannot be used to remove stamps from covers. Common water is normally used to remove older stamps, and on non-fugitive ink stamps, you also shouldn't see any evidence that it was used. So when you say it was obvious that water was used, my answer is, of course, that's the only way to do it.

Others should chime in on the best way to work on known fugitive inks.