Pocket Thread Gauge
Here's a handy gauge for identifying metric bolts. It is made of stainless steel and the exact same size as a credit card.
Measures length up to 85mm
Identifies diameters M4, M5, M6, M8, M10 and M12
Identifies thread pitches 0.5, 0.7, 0.8, 1.0, 1.25, 1.5 and 1.75
This tool is at home in your toolbox and also a handy thing to have along on any ride, as it takes the guess work out of identifying bolts and screws.