Does it have a watermark or not?

Ok this kinda sorta bugs me - when I have to identify a stamp by watermark, I don’t put a stamp with the watermarked varieties unless a watermark is clearly visible. BUT what about the stamps that don’t have a clear watermark? Are those REALLY from a sheet printed on unwatermarked paper OR are they printed on watermarked paper but that stamp was printed on a section of paper without a watermark? Sometimes the watermark paper just looks very different - slightly mottled in appearance so I can be fairly sure which copies are the unwatermarked types. Sometimes the papers are really similar. 

whew - what to do, what to do …