Use a level and tape measure
Place the level horizontally against the roof surface
Measure 12 inches from one end of the level along the roof
Measure the vertical rise from the roof surface to the 12-inch mark
Write the slope as rise over 12, such as 4/12 or 6/12
Use a roof pitch gauge for a direct reading
Use a smartphone inclinometer app on the roof surface
Measure from the attic by finding the rafter and calculating rise over run
Use a framing square to compare rise and run
Confirm the measurement in multiple spots on the roof