Remove the toilet
Measure from the finished wall behind the toilet to the center of the closet flange
Measure in a straight line, not along the baseboard
Use the finished wall, not the baseboard or trim, as the starting point
If the toilet is still installed, measure from the wall to the center of the bolt caps or flange location as closely as possible
Standard rough-in sizes are usually 12 inches, 10 inches, or 14 inches
Confirm the measurement on both sides if the wall is not perfectly square
Check for obstructions before installing a new toilet