How to identify screw specifications

The screw type is mainly identified by three parts.
1 head. This is very easy to identify. With the naked eye can intuitive judgment, such as a head, or other special type.
2 diameter. The eyes, it is hard to judge the very accurate, this time we often use vernier caliper to measure the length, pay attention to the diameter is and the diameter of the tooth length.
The length of 3. Also with vernier caliper, pay attention to the length of the length of the header is not included.
Of course, this is just a relatively accurate measurement, if fairly accurate data, to use professional equipment to measure.