As stamp collecting becomes ever more popular as a hobby and also as a financial investment buyers need to be more aware of buying stamps that are not all that they seem. I am a keen collector of Queen Elizabeth II  wildings and have spent a small fortune on the different types. The hardest part of collecting these is distinguishing the different types of water marks and unless you have a quality water mark detector, you have to rely on the honesty and expertise of the seller. Even unmounted mint stamps with full gum can pose problems in identifying a Tudor Crown from a St. Edwards' Crown.On used stamps I do use lighter fluid to expose the water mark but this is only done as a last resort and does not affect the gum on the stamp or affect the quality. I would never use this method however on mint stamps that have cost a small fortune, only because I would kick myself if anything happened during dipping the stamp in lighter fluid.The hardest stamps to check are mounted stamps which have the hinge covering the water mark.I find the best way is to invest in a good magnifying glass and a Morley-Bright Inst-a-tector. Then check the stamp in full sunlight as I find this helps greatly in this process.
So, be warned that sellers might themselves just guess as to what water mark lurks under that hinge remnant or that gum!! I indeed bought a wilding from a reputable dealer only to find that my Tudor Crown was a Multiple Crown.