I sell digital photos on my home page, and the most common question I get is, “Are these pictures 300 DPI?” Unfortunately, I usually can’t answer definitively, because they don’t tell me the size they’re going to print at.

DPI (dots-per-inch) depends on two factors: the resolution of the picture (“dots”) and the size of the image as you will print it (“per inch”). My website shows the resolution for every picture at the top of the page, as part of the pictures description.
For example, this picture has a resolution of 3072 x 2048:

If you wanted to print it at 3×5, it would be 614.4DPI (3072/5=614.4). Vertically, it would be 682.6DPI (2048/3), but you’d need to crop it vertically to get the 3×5 proportions, so the DPI will be the same once it’s cropped.