Weapon Body Language
An experienced operator can tell you at glance if the person holding the weapon knows how to handle the weapon that they are holding.
The way in which a person walks with a weapon will reveal a lot about them and their level of training.
If a man walks with a M4 close to his chest with an index finger up along the trigger housing but not touching it, I will assume that man is trained on that weapon. If a person has a weapon slung over their back with the barrel up, I'm assuming that person is not confident with their weapon or properly trained.
Body language has a lot to do with how you hold your weapon. Most military personnel who have travelled, eaten and slept with their rifle make the rifle appear to be an extension of their body. Military and police officers are very comfortable with their weapon and their body/weapon movement reflects their level of confidence.