Use irfanview (www.irfanview.com) It is a free image viewer, very fast and lightweight. Once you open the image in irfanview you can see the color depth on the bottom of the program (width x height x depth bpp), to change it go to Image - Increase / Decrease color depth.
I am sorry, but that is completely wrong. The color depth, also known as bits per pixel (bpp) is the ammount of unique colors in a picture. With modern image compression techniques an adaptive palette is used, which is usually based off 24bit color depth (even if only using 256 colors of that range).