Since you have already taken it out, that's easy. Measure the length (minus the head) and the diameter (outer part of the thread, i.e. the smallest hole the bolt will fit through) rounded up to whole millimeters. If that diameter is 6 mm, it's an M6 bolt. 5 mm = M5. Kindly post the numbers here, I'm also interested.

If any part of the bolt is non-threaded, that's also worth measuring.