I've seen Blood Diamond on TV but I've missed some scenes so it makes me confused. When Danny Archer told Solomon "Time I quit smoking anyway, huh, Solomon?" Why would Solomon knows it is a signal of attack?
Earlier in the film, the following exchange takes place between Archer and Solomon (from a transcript of the film):
Archer: I've gotta quit smoking, huh?
Solomon: Why don't you?
Archer: I'll tell you what, Solomon. You find me this diamond, and I'll quit right then and there, huh?
So, Archer will quit when he has the diamond. That's his "end game". Later, towards the end of the film, the dialogue you refer to takes place:
Archer: Hey, boet, have you got a smoke?
Guard: Smoking will kill you. Sorry.
Archer: It's all right. Time I quit smoking anyway, huh, Solomon?
I took this as Archer's way of telling Solomon that the "end game" has arrived. If they don't get the diamond they're dead. Even if they do find it, they're probably dead.
So we see a pause, as some sort of realisation comes over Solomon's face. He initially says he can't find the diamond and realises they'll all be killed (including his son), so he immediately backtracks and digs for it. By acting overly excited (with Archer helping) they manage to distract the guards enough to disarm them and make their escape.
So to answer your question: It's not that Solomon knows it's a definite sign of attack, as much as it's a sign that the end has approached and he needs to do something.