1. Unfavorable information (true or false) about person A is presented.
2. Therefore, (explicitly or implicitly) any claims about to be made by person A should be dismissed.
"Don't listen to anything Steve may tell you, he's a socialist."
"Before you listen to my opponent, may I remind you that he has been to prison."
Practically speaking, poisoning the well is a form of ad hominem, and as such, one should follow the guidelines of analyzing an ad hominem to determine if it is being used in a fallacious manner. This essentially means questioning the relevancy of the attack on the claims presented by the person for whom the attack was directed against.