Well, if it's in tweezers, and the tweezers look really bright white, that's sometimes a sign. If it looks like it has a lot of light shining on the stone, or it's photographed against a dark background, that's also a sign. Some species are also pretty dark, like blue spinels, and green tourmalines. Of course, there's exceptions, but I find some species tend to go black on you unless you find an exceptional stone. Tourmaline, for example, sometimes grows with a very dark axis in its crystal structure, and if cut with the table along that axis, it can be very dark. That's why I always shop for tourmalines that have an "open c-axis" as they call it. In other words, the c-axis (the sometimes dark axis) is the same tone as the usually lighter a-b axis. Stones that are cut very deep can also be very dark as well.
I am hyper sensitive to dark stones, and I find that I prefer tones that are medium and lighter in tone. That helps me to avoid mass extinction (and I'm not talking about dinosaurs). LOL!
Here are a couple things that you can look for short of asking the vendor himself/herself:
1. Very bright and reflective tweezers mean lots of light is used to make the stone lighter in pictures.
2. Look for dark shadows under the stone
3. Take note of the background colour