While it's a very simple measurement to make, people often misrepresent the wheel size for their bicycle because they're measuring the wrong thing. The wheel size for a mountain bike -- or any other type of bike -- is a combined total of the rim diameter and the thickness of the tire itself. Many mountain bikes have a 26-inch "wheel," -- again, meaning the rim plus tire -- but you can determine for sure using one of two methods.
Look on the side of the front or back tire or the rim for a set of numbers. Often, you'll find a set of numbers that reads "26" X 1.5" or something similar; this indicates that the wheel is 26 inches total, with a 1.5-inch tire. This is by far the easiest way to measure the wheel size, since it's spelled out in no uncertain terms right on the equipment. But if you don't see any numbers, you can still measure the size yourself.
Hold the bike or tire and rim combination in place, or better yet, have a friend hold it for you.
Place a yardstick up against the outer edge of the mountain bike tire.
Stretch a measuring tape across the length of the rim and tire, placing the measuring tape right on top of the axle in order to measure the wheel at its widest point. Push the end of the measuring tape up against the yardstick so that you know you're getting an exact measurement at its end.
Read the end of the measuring tape that is opposite the yardstick. If it is a standard mountain bike, you should find that the wheel is 26 inches across, though it could also be a 29- or 27.5-inch wheel.