How could you tell unless it was Red Dot or Green Dot?
They tear down shells all the time on web sites and magazines and you get a lot of "it looks like X powder". There is no way to tell and no advantage to know because as richdeerht said many commercial powders are not available to us.
Many years ago, I bought 2 lbs of powder from an estate. It was IMR, but the brand unknown. I put a very small amount in an envelope with a letter sent to DuPont -manufacturer at that time. They responded, saying there is no way to identify powder unless they do a bomb test.
They put a certain known amount of powder in a sealed container, place it in a tub and explode it with instruments attached to record the percussion.
I don't know what powder they use. However I have for example taken the Remington ammo catalog and found the velocity of their factory round. Then went to my reloading data and found the powder velocity that was the closest to the Rem data in the catalog... Many times you will find that your factory ammo shoots so well you want to duplicate it.. So keep a Rem, Federal, Win gun or ammo catalog with your reloading books. Just saying what I do...