People bring you tips for a range of reasons, many of which have nothing to do with helping investigative journalism or exposing wrongdoing. The same may be true when you approach a source. Personal grievances, circumstances or beliefs may colour what they say, leading them to exaggerate some aspects or stay silent about others. Some sources may be over-eager to be helpful, and give answers they think you want to hear. Your background research on the source may shed light on some of this; your observation of how they behave while they talk to you will also help.
People sometimes make honest mistakes and forget details. For both these reasons, you need to verify everything through a third, independent source. Evidence should all point in the same direction (but only rarely will it be exactly the same) from two sources who have not learned it from one another. If you cannot find a second source, or there is simply no time, you may have to confide in your audience that, ‘it was impossible to confirm the statement.’ Too many unconfirmed statements, claims and allegations in a story will weaken it and compromise your integrity as an investigative journalist.
But suppose your second source provides conflicting information rather than confirmation? In this case, you should inform your audience of both positions, or integrate the conflict between them into the story: ‘The interior ministry said armed men crossed the border; the defence ministry described them as unarmed.’ If some details do not fit into the story you were trying to tell, you simply cannot ignore them; your own credibility and professionalism are at stake here. Journalists with a distinguished track record and extensive contact networks, such as Seymour Hersh, may sometimes have relied on a single source. Very few are in that league.
Whoever you will talk to, at the most basic level, you need to find out whether the person is who he or she claims to be. Can they prove where they work, their address, their family details, military record, passport, ID or driver’s licence? If a source has a history of crime, personal difficulties, mental illness, financial problems, violence or fraud, you will need to be particularly sceptical about what they tell you. If a source resists, there are probably strong reasons why he or she is hiding the information, and you need to factor this into your judgment about whether you can trust the information they provide.
Only when you know what you are looking for will you be able to assess the quality of what you get. Does the source provide a complete explanation or set of evidence? Could you piece it together in any other, equally plausible way, and come to a different conclusion? Where are the ‘holes’? Is the source’s experience likely to be representative of experiences in his/her community? Is it up-to-date, or did it happen so long ago that things may have changed and details misremembered?