Whether it is a commercial or a matrimonial matter, there can be a need for an investigator (costly but effective) as noted in this recent landlord/tenant case:
NY POST - Nolita landlords say tenant earned $500K over 4 years by illegally renting out apartment as a NYC ‘hotel room’ - used private eye posing as tourist to out sublet
NY POST - Nolita landlords say tenant earned $500K over 4 years by illegally renting out apartment as a NYC ‘hotel room’ - used private eye posing as tourist to out sublet