What is the indication that the last ammunition round has been fired?

Answer 1

Depends on which weapon you're referring to. Most (but not all) service rifles have a bolt hold open device which keeps the bolt locked to the rear when the last round is fired. The Garand had the ammunition which was loaded into a clip, which was then loaded into a magazine, and that clip would spring out when the last round was fired (although unfortunately it also gave away this fact to the enemy sometimes).

Weapons such as the Kalashnikov do not have any such feature, and you may not know you're out of ammo until your weapons stops firing, or you might grind down the follower stops in the magazine, which will effectively hold the bolt to the rear when the last round is fired (although it will slam back forward once the magazine is removed).

Another prescribed (although not commonly practiced) method is to load three tracers into the magazine first - these will be the last three rounds fired, and will indicate that the weapon is out of ammo. However, this also works both ways, and lets the opposition know you have to stop and reload, as well.