Photo: Michael Jung (Used under license)
Human beings are built to thrive in groups, working with others in harmony in order to make the most of the talents and abilities in order to increase the chances of success. Within these tribes, smaller teams naturally form, making tasks more efficient as large jobs can be divided into smaller chunks. Although uncertainty is allowable and healthy in small doses, performing tasks such as fostering constructive competition between members of the team, it can be cruel and destructive in too large of an amount. Therefore, one needs to learn how to calmly and confidently navigate the waters around doubt in order to prevent their relationship from breaking down.
Firstly, if there is no evidence of any wrongdoing except for a nagging voice in the back of the mind, the only plausible and logical action to take is to further probe without letting paranoia take over. Casually asking indirect questions to mutual friends is a responsible and non-invasive task to undertake if one is careful not to arouse suspicions, but checking someone's phone or computer behind their back is never okay. In fact, the latter undermines trust and may even aggravate any feelings of doubt if certain findings are taken out of context.
If subtlety is not a strong suit, talk to the person directly without any aggression or hint of confrontation. Be sure to carefully articulate any worries and doubts in a non-accusatory way in order to prevent an unnecessary fight. Keep in mind the other person may feel hurt, so being straightforward and somewhat blunt (the equivalent of ripping off a Band-Aid quickly) is a must to maintain a short conversation. Also, allow them to defend themselves and try not to interrupt or bring up past events unless they are extremely relevant to the case. If worse comes to worst, getting away from the situation for a while could be the best strategy.
Lastly, and perhaps most importantly, learn to let unfounded worries seep away. If a relationship is still intact after thoroughly communicating and low-key investigations do not come to fruition, abandon whatever doubt there is left and accept that it will likely fade over time. Do not let it become consuming or seep into everyday interactions with the person. This is not to say one should ignore their gut feeling and trust blindly without evaluating the circumstances, but it is certainly easier and happier to keep an open heart.
All in all, while doubt can be positive and can even bring people closer together or make tasks more efficient, too much of it is unhealthy and detrimental to any relationship. The key ideas to keep in mind are to try to discover evidence or have a meaningful conversation with the other person before letting fear and uncertainty loose, and ignore doubt when nothing suspicious or incriminating surfaces. Being open minded and accepting of others' choices and privacy is essential to make any relationship last. Doubt may surface at any time, but it does not have to reign or wreak havoc.