The pin in the picture looks like a 5/16x14 piloted joint. As already mentioned, the diameter of the pin is the first measurement, in your case it looks like 5/16. The number of threads per inch is the second measurement, in your case it looks like 14 (14 threads per inch)
Sometimes the pin may not be one inch long, in this case you would count how many threads per half inch and then multiply your answer by 2. So if you count 7 threads per half inch, you have a 14 thread per inch joint.
Make sure your start your count on a thread and then go up a half inch and finish your count on a thread.
The most common joints are:
5/16x14
5/16x18
UniLoc Quick Release
3/8x10
Other notables are:
Radial
3/8x11
and Cuetec/Scorpion joints which I believe are 3/8x13?
You can also see a list of some of the most common cues and their respective joints here:
http://www.predatorcues.com/images/joint_selector.pdf
Feel free to email me or call me if you need any other help with this.