Shylin, Hosman is correct, those "signatures" can be very small and very elusive to "see".
Sometimes it requires a magnifying glass, ..... holding it in just the right position, ....... in just the right type of light (bright Sunlight, fluorescent, natural light, etc.) ...... and/or a lot of luck.
One has to "check out" every itty bitty what-looks-to-be an "imperfection" in the glass because it just might be ..... "the signature".