[Gollum]

Also note that if you want to play in a very realistic manner, when someone attacks, it can be hard to know if he is doing a normal or an all out attack.

I never tell to my players what maneuver their foes choose. I only describe them what they can see (The skeleton jumps and tries to strike your head with a vertical blow) ...

Thus, combatant B can never be sure that A isn't able to defend himself after his attack. That avoid this kind of situation where choosing an all out attack becomes totally safe.

Of course, if the player of an experienced warrior asks me: "Is it a normal or an all-out attack?", his character should have some chance to know it: he is experienced enough to guess it just by looking at the manner of attacking. So, in that case, I would secretly make a secret combat skill roll based on Perception and, if successful, I would tell him. But only if the roll is successful.