Before speaking about something you feel needs correcting (aka criticism), engaging in gossip, etc...:
"Is it true? Is it beneficial? Is this the right time and place to say it? Am I the right person to say this? If you can honestly answer Yes to all four questions, go ahead with your comments. Even then, though, you have to look at their results. If you see that they weren’t actually beneficial or timely, learn from your mistake. That’s how your discernment grows." —Thanissaro Bhikkhu
my addendum to this has been: Is it loving? Is it Kind? Is it needed? Is it useful and to whom? Will it bring peace, clarity and closure in the long term? Is it respectfully phrased? Is your ego attached to the outcome in any way? i.e. do you need to be right about something? Are you saying this because you need to say it, or because someone needs to hear it? Would silence preferable?