Use a level and tape measure
Place the level horizontally against the roof
Measure 12 inches along the level from one end
Measure the vertical distance from the roof surface to the 12-inch mark
Record the rise over 12 inches as the pitch
Express the pitch as rise over run, such as 6/12
Use a pitch gauge or angle finder if available
Convert the angle to pitch if needed
Measure from the attic underside of the roof if exterior access is unsafe
Repeat the measurement in multiple roof areas for accuracy