Hilchos Lashon Hara chapter 10, section 2

These are the seven conditions:

a) You must see what happened yourself, and not just hear it from others. The exception is if you later found out for sure that it is true.

b) Be very careful not to decide right away that it was theft, cheating, damage or the like. First think it through carefully, and check whether it really fits these categories.

c) First, gently correct the person who did wrong. This might help, and he might change his ways. If he does not listen, you may then tell people what he did to the other person. (If you know he will not accept correction, see paragraph 7 below.)

d) Do not make the wrong sound bigger than it was.

e) Your goal must be to help, as explained below in paragraph 4. It must not be to enjoy putting the person down, or because you already hated him.

f) If you can reach the same good result some other way, without speaking lashon hara about him, you may not speak about him.

g) Your story must not cause him more harm than halacha allows. That means no more harm than a beis din (Jewish court) would cause him if you testified against him there. This is explained later in Hilchos Rechilus, chapter 9.