You shouldn't pay to watch your spouse
I do not believe that it is ethical for a wife or a husband to hire a private investigator to watch his or her spouse. To do so means that there is no trust in the relationship. To pay someone to watch your spouse would be a waste of money.
One the other hand, when the spouse who is being watched has found out, if she is not cheating, she would think that you do not trust her and perhaps she may begin to date the private investigator if they have feelings for each other. So, the person you hire could be the one dating your spouse.
Some persons have had good reasons to hire private investigators to watch their husbands or wives. Sometimes it is not a matter of trust, it is a matter of knowing the truth. Oftentimes, that is the only way for a spouse to find out the truth. You may consider this a waste of money, but if a man or woman wants to know the truth and wants to hire a private investigator, he or she is not wasting his or her money.
As I write this letter, I am thinking of a dear lady who cheated on her husband for many years and he never knew, and she was living in the same house with him. But, there came a time when the man with whom she was having an affair wanted to marry her, and she decided to divorce her husband. When the papers were served on him, she had to leave his house and he swore that he would kill his wife. The divorce came through, and for many years he did not know where she was living.
Perhaps if her husband was alert, he would have known that something was not right in their relationship. He was not supporting her well, and yet he wanted to eat costly meals. The 'bunna man' was providing everything for her, including the very food the husband was eating.
A man has a right to find out whether his wife, on whom he has spent or is spending lots of money to support is cheating on him. And private investigator is trained in this area of work. He or she is a professional and knows what to do without being detected. It would not be a waste of money to pay him or her for their service.