It should say on the rim, on the surface facing the axle. It is probably by the seam, which is opposite the valve hole.

If it IS a 27, you can take any of the widths-1, 1-1/8, 1-1/4, or 1-3/8. Not so for 26 or 28! there's at least eight different rim diameters called 26, and at least two 28s, so tell us what it says.