I guess my simple rule is that once one starts explaining logic or English then it is time to give up, not that I follow this rule all the time. But are there other helpful signs?
Yeah, there is an extremely helpful sign; namely the handle of the poster. Except for the very newest posters, the people who will be a waste of time in the next thread emphasizing logical discussion of law, history, science, the constitution, current events, etc. are the same people who wasted your time in the last four such threads they participated in.
If you truly want to avoid wasting your time, you don't have to wait for the all caps, multi-colored, large font posts, or the heartfelt arguments that jump shark. You know those things are coming.
But those things are also part of the fun. Those ridiculous posts allow you to post paragraph long dismantlements with relentless, attack proof logic; perhaps with a little faux testiness or snark. And as you suggest, the truth does get in front of the lurkers.
Under a government which imprisons any unjustly, the true place for a just man is also in prison. Thoreau: Civil Disobedience (1846)
The apathy of the people is enough to make every statue leap from its pedestal and hasten the resurrection of the dead. William Lloyd Garrison.
If there is no struggle, there is no progress. Those who profess to favor freedom, and deprecate agitation, are men who want crops without plowing up the ground, they want rain without thunder and lightning. Frederick Douglass