You need a PI OR a disinterested 3rd party to prove adultery . Fault would have more of an affect on the person asking for alimony , a lesser or no award if the person requesting it were at fault for the break-up of the marriage . Fault is a consideration for whether or not support is ordered but shouldn't really effect the amount but it COULD effect the duration of any award . The amount depends more on the assets/income of the person that would be required to pay .