Two ways, either by the tire size, which means knowing or measuring the overall diameter, and working back about 6-8mm for the bead seat diameter and comparing it to standard sizes here. You also want to know the inside width, but this isn't as critical since each width can accept a range of tire widths, running form about 1.5x to over 2x the inside rim width.

For wheel building, you need to know the spoke hole diameter, or if using a spoke calculator the ERD (effective rim diameter). The easiest way to measure this is to drop a nipple into a hole, measure the overall diameter to the edges of the rim. lay a straightedge across the rim and measure the drop to the nipple. Subtract 1-2mm from this to have an ERD between the slot and top of the nipple.

Whenever measuring rim diameters, it's best to take two measurements at right angles in case the rim is slightly. The round diameter is the average of these.