Usually I'll hear something from one team about two other teams discussing a trade, and from there, I'll reach out to other teams not involved, then agents of the players, and then bring all the information I can to the to teams that are actually involved, and hope they confirm all the intel. If they don't, I don't report. If they do, press publish.
If you don't expect team people and agents to lie every now and then, I think you're going about this naively. There's always a chance for misinformation, but that's why you spend years cultivating relationships, going to off-the-record dinners and drinks and spending time with people so you can develop that trust, and even learn to know when people aren't telling the truth.