This photo depicts how I measure a bicycle's trail. First I suspend a plumb bob (use a key tied to a length of string) from the very end of the threaded side of the front-wheel's Q/R skewer. Then I hook a string over/around the fork steerer or stem quill, and stretch it down parallel to the steerer and the upper fork legs to the ground, where it's held against a ruler. Measure the distance between the stretched string and the plumb-bob and you've got a rough trail value. Make sure the bike is positioned on level ground with front wheel straight, and is as vertical as you can make it.