Sincerity can be quite an overplayed factor with cheating spouses since the cheater always thrives on the trust of the faithful partner. The faithful spouse is more or less inclined into believing their partners recounts since this gives them both a sense of comfort as much as it guarantees their security. It is thus quite normal that a lot of people do not know how to go about finding out if their spouse has breached their fidelity. The investigation of such a case should, however, be conducted with as much caution as can be afforded seeing as your efforts will be quite amateur. Communicating your suspicion in loud actions will very likely be your downfall as the obvious cheating signs that were once quite visible will be hidden by the cheater taking more care in their habits.
Trying to understand the problem in the first place will give you a foothold in the determination of your spouse's illicit activities. Since you are inclined to trust your partner's statements it is prudent to know that this is the unfair advantage that they have over you. The redundancy of statements such as "You can always trust me" tend to veil the situation at hand making it difficult to catch the cheater. Therefore to answer the question: is my spouse cheating; will necessitate that you take into concert your hindsight knowledge and keep tabs of all the obvious signs.
Discretion dictates that you confront the cheating spouse with hard evidence upon expressing your suspicion and always keep a look out for the fallacious statements that they will make. They are likely to insist on their innocence even upon presentation of such evidence.
Overlooking the spouse's faithfulness only serves to frustrate your efforts in finding out if at all the agreement between the two of you has been compromised. Facing the ugliness of the existent truth cools down some of the pain you are likely to experience. You are likely to put an end to your problems if at all you incorporate the tell-tale signals into your judgment thus dealing with the cheater clinically.