My Nikon D3100 as well as its 18-55mm kitlens and a sigma 70-300mm zoomlens was accidentally dropped on the ground while in a camera bag, by someone who did not know it was a camera and mistook it for a sleeping bag, because I had a shopping bag wrapped around it (to protect it from excess dust). Because I was not present at the time it happened and only found out afterwards, I have no idea what height it was dropped from, but I'm guessing it was about 50cm. I could find no external damage and it still takes good photos, but since it is my first DSLR, I have no idea what to look for. Another question that has been bothering me is could this cause banding or loss of quality in my photos. Any help would be greatly appreciated.
Yes. It is possible to damage a camera from a fall from any height or even just bumping in to something too hard. It isn't particularly likely, but can happen. There doesn't have to be visible or significant damage either. Momentum of internal lens elements during a fall can result in knocking things slightly out of alignment which could result in slight losses of sharpness or introductions of aberrations on parts of the image.
If you don't notice anything in your images, it probably isn't anything major, but you can't tell for certain without having the calibration of the lens checked to make sure it is working fully to spec. Practically though, if you don't notice a problem, it isn't worth worrying about.
A 50cm fall on hard ground can easily damage a lot of equipment. However, you wrote "in a camera bag" wrapped in a shopping bag. That means some cushioning of the deceleration. Which is one of the intended purposes of a camera bag.
Now you apparently had the body with a smallish mounted lens(?) and a larger lens in that bag. Typical camera bags will separate the elements in them from banging up on one another.
So for a typical bag and situation, you very well might be fine. How did you "find out afterwards"? Being told, or some visible damage somewhere?