Exactly! During the Thanksgiving scene, Alma lets's Ennis know that he can't fool her anymore with his "once burned" comment. Not being truthful to her about the relationship, after all those years clearly contributed to her anger at Ennis and Jack both. She felt put in a position of helplessness, unable to concieve of anyway to compete with this. She had to realize why their sex life went the way it did further causing her to see that it was Jack in Ennis thoughts while making love, not her. Tying the note to the fishing line was a clever deed,proving that she was Not a complete fool. She needed *proof* of her suspicions and she got it. This realization coupled with what she saw outside their door confirmed the true nature of her husband's preference. Jack. By the way, does anyone wonder about Monroe's behavior during the Thanksgiving scene. Alma is screaming at Ennis in the kitchen while he sits seemingly idly by watching skating!!/ What' is up with that? Is he wimpy or what? The guy never left his chair to see what all the commotion was about. 
Great points. aintfoolin!
I personally suspect that long before the divorce, Alma and Ennis treated everything as an open secret between the two of them. Ennis probably realized that Alma must have suspected or perhaps even knew more than she let on. But as I said in a previous post, he did not care -- so long as Alma played the game of noting bringing things out in the open.
As for Monroe, he probably did not hear anything until Alma started screaming. I wouldn't be surprised if Monroe knew something. After the thanksgiving scene, it is possible that Alma told him, if he had not been told before.
KNowledge of this beyond Ennis, Alma, and Jack would have guaranteed that there would be no separate peace. A separate peace in such a situation would have meant a loss of face.
