Take a 12" long board, 2x, 1x, etc.,
place it on the roof with one end pointing to the gutter and the other end pointing to the upper end, than leaving the stick pointing toward the upper end firmly on the roof lift the end toward the gutter up until it's level,
measure from the roof to the bottom of the board and that's your pitch.
If you take the time to make sure it's as level as possible your read will be fairly accurate, usually with in 1/4".
Like Ed said, Certainteed 'I believe' has spec's for a 2 1/2/12 pitch but most stop at a 3/12 pitch being the lowest for allowing shingles.