Maybe the pilots could also come to the gate before the flight and justify their competence by showing you their logbook, inviting type-specific and regulatory tech questions and maybe showing you a copy of their training record and last sim write-up. Then you would be able to make an informed decision.
Sorry for the sarcasm but you have to draw a line somewhere and trust the regulation. The operator will be required to roster within the confines of the applicable flight and duty time limitations which are freely available to see.