I am purposely not going to go into a lot of detail here because most of this information about stamp gum can be found in the Scott catalogs as well as numerous other philatelic publications. And I have no desire to provide and exhausted study of the different kinds of gum used and what issues they are used on. That would consume many pages of effort.
Up until the 1980s, the USPS (and its predecessor, the USPOD) issued postage stamps with water activated gum. Moisture caused the gum to be sticky, holding the stamp onto the envelope. The gum is very shiny. Looking at the back of the stamp, it is very obvious if the stamp has gum or not.
In the 1980s and 1990s, the USPS experimented with a variety of different gums. Traditional water activated gum still had a tendency to curl in humid environments. Or sometimes the panes of stamps could become lightly stuck together. The experimental gums were an attempt to improve that.
These new formulas for gum made the appearance very dull. Take one example, Scott #1582 (there are many other examples). It exists with both shiny water activated gum and the experimental dull gum. Copies of both gum types are very common and readily available.
How do you know if a stamp has gum or not? Because the dull gum has no shine to it, it looks like the gum is missing. You could moisten the back of the stamp in a small area and see if it is sticky or not. But that kind of defeats the whole purpose of having a mint, never hinged stamp.
The only way I have been able to tell is through feel. I know, you are supposed to use tongs and not finger your stamps. But this is the only way I know if a stamp has gum or not.
Lightly run your finger across the back of the stamp. Mint stamps that have dull gum have a “smooth and soft” feeling to me. Stamps without gum do not feel as smooth. As a comparison, run your finger along the back of a used copy of #1582. Then run your finger along a known mint copy with dull gum. You should feel a difference.
Sometimes a customer will return a stamp saying that it has no gum. I have to explain that it does have gum – dull gum to be precise.
By the late 1990s, the USPS was turning to self-adhesive stamps. All of the experimentation into different water activated gums did not go far. Today, all stamps are self-adhesive and it’s very obvious if they have gum or not.