To make it clear, it's essential to differentiate a watchdog from a guard dog. A watchdog is likely to sound an alarm whenever he spots an intruder or danger, and not more than that. There may be cases in which the lab may try to attack, but they may take you friendly once you give them a treat. On the other side, a guard dog will not only sound an alarm, but he can also go on an attack.
Labradors are incapable of attacking someone as they are vocal and can communicate well by barking. So, after so much research, there's no proper training for your home Labrador to become a watchdog, as they are intelligent enough to differentiate between friendly people and threats.