Determining an agenda

Somehow, the media often knows news first.
Much of the reason behind that is people tip us off. Reporters are frequently getting calls from the public letting us know when something's going down.
But we can't forget to ask ourselves why they call.
For big things, like insider information or crime or legal troubles, we have to think what the caller's motive is for tipping us off. Why does this person want the public to know?
Making a phone call or writing an e-mail is not difficult, but people generally are lazy and won't put in the effort unless they think they get something out of it.
Not everyone, I hope, has a motive. I like to think there are some nice people in the world who believe in the public's right to know and in aiding the media, but I am not so naive that I think everyone is just trying to help us out.

So... how to tell the difference between a friendly heads-up and someone trying to stir up trouble or take someone else down?

I make every attempt to research the caller if possible, and think about what they have to gain from public knowledge of their tip. Sometimes the only solution I come up with is that they just want people to know, but sometimes there is definitely a little sinister something in a caller's mind when they pick up the phone to dial.