The landlord should be able to determine more about the prospective tenants at the viewing. Address any issues which are of concern at the viewing, especially if the prospective tenants show a definitive interest in the premises. Then perform all of the proper screenings on all adult occupants -- there is no actual formula to pre-screen tenants prior to meeting and obtaining a full rental application with documentation for review and verification.
In regards to the all adult occupants - screening, often I've had them (when potential renters come as a couple) fill out the information on one application with her info being noted and his. In previous circumstances this worked, I'm wondering if I have just been lucky up till now. I'll ask this person for a pay stub.