You need a UV (UltraViolet) lamp. To be precise, you need a shortwave UV lamp (many countries use long UV as opposed to short). Most, but not all, tagging omitted errors, are rather scarce. I once checked several thousand stamps for my brother and not a single one was untagged.
Jack, all of this UV stuff is sometimes complicated, especially if you collect foreign counties. or the US however all you need is short wave ( SWUV ) for tagging. Long wave ( LWUV ) is useful if you like to distinguish between regular and high brite paper or when looking for repairs or other irregularities