Determine the focal length of the device you're using. This should be located either on the device or in the owner's manual that came with the device. For example, on a telescope the focal length is usually 2,000 mm or less.
Determine the focal length of the eyepiece on the device that you're using. Again, this information will be printed either on or near the eyepiece, or in the owner's manual. On a telescope, for example, the focal length of the eyepiece is typically between 4 mm and 40 mm.
Divide the larger number (the focal length) by the smaller number (the eyepiece focal length), and the result will be your magnification. For example, a telescope with a 1000mm focal length divided by a 10mm eyepiece focal length will produce 100x magnification. This procedure works for any device---simply take the focal length of the viewing device divided by the eyepiece focal length to determine your magnification.