* Sleeves: Bachelor's gowns typically have long pointed sleeves that are open at the wrist. Master's gowns have longer sleeves that are often closed at the wrist with a slit at the very end. Sometimes they're described as having "longer, closed sleeves with oblong slits." Think of it like the difference between a long glove and a sleeve reaching to the wrist with a small opening for the hand.
* Hood: The hood is the most significant differentiator, but it's not always immediately obvious. While both will have hoods, the *shape and lining* of the master's hood is different. Master's hoods are generally larger and have a different shape than bachelor's hoods, and importantly, a different lining color or a different arrangement of lining colors representing the field of study. (The hood lining, specifically, is what shows the specific degree and often the institution). This is the most reliable way to tell the difference from a distance. A bachelor's hood will be much simpler.
* Material: There might be subtle differences in the material or quality of the fabric, but this isn't a reliable indicator.
In short, look closely at the sleeves and, especially, the hood. The longer, potentially closed sleeves and the more elaborate hood are the key giveaways for a master's gown.