In general, adherance to company policy, applicable FARs / opspecs, and a good sample of VFR/IFR procedures. If it's a SIC check, they are evaluated as a SIC, in addition to a scenario where the PIC becomes incapacitated. For a PIC check, including companies approved for single pilot ops, there is a crew evaluation as well as a single pilot scenario. A good starting point would be the Comm or ATP PTS, including emergency procedures and multiple flavors of approaches.