How did people gather evidence to substantiate an allegation of adultery?

The difficulty of proof has given rise to a whole range of investigative services as well as inventive deception on the part of parties who wanted to divorce in the days before no-fault.

In the old days, private detectives who stalked the suspected adulterer worked in conjunction with divorce lawyers who had reputations for success in these then-difficult, legally taxing cases. This involved surreptitious surveillance of the comings and goings of a spouse.

The problems of proof -- as well as the cost of doing it, financially as well as emotionally -- make it likely that other grounds for divorce, if available, should be alleged instead of, or in addition to, adultery.