Welcome to the forum! When I check a roof's angle or cut, I carry a mini protractor, or angle finder.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=65691
I cut the level and plumb bubbles off, now it's about 3-1/2",to keep in my tool apron pocket. You can also hold your 12" level up, with tape measure plumb at the end. Line level up with horizontal, extend tape to get cut of roof.
To check a house for an estimate, I measure it at exterior, or sonic tape at interior. If a lock-out, I use binocs to count the shingles at the eaves horizontally, 1 per 12", from the vehicle. Really hard with architectural shingles........ Be safe, G