Under the user name, there is a user title shown. The rank is decided from the numbers of counted posts made, according to this list:
Recruit - 1
Private - 10
Corporal - 25
Sergeant - 50
Second Lieutenant - 100
First Lieutenant - 200
Captain - 300
Major - 500
Colonel - 800
Lt. General - 1200
General - 1700
Field Marshal - 2500