Hi there,
I guess the red dot sticker is the only way to clearly tell from the
outside of the unit if it has the mod on.
You can also look at the bottom of the unit and you will find four
extra screws holding the BS add-on card. The card is small so they`re
the four screws closer to each other.
I don`t know anything about serial number identification.
By opening the top lid of the preamp you can clearly see if it has a
Billy Sheehan Mod add-on card, but you have to access the inside of
the unit. (I can send pictures of mine if you want).
Hope this helps,
gaston