Fanxiyi, a rather unhelpful answer is that the aircraft design should have considered the minimum crew and workload aspects (e.g. CS25 1309/1302), thus the flight test task is to check that the aircraft operation is as designed.
If the evaluation involves modification of an existing type, e.g. re-equipped flight deck, then the task could be based on a flight deck evaluation of the equipment layout and in-service operation (‘same-type’ approval). But again the modifications should have considered these aspects.
An avionics vendor can sell a system which enables ‘single pilot’ operation, but this might not consider other essential aircraft systems which are unaffected by the avionics, e.g. gear and flap operation, fuel management, pressurisation. Of course both normal and abnormal operation has to be considered.
I was involved in the workload assessment of a new aircraft type at the time of the transition from a three-crew to a two-crew flight deck. The politics at the time may have biased the amount of work required, but even so the flight task alone was significant; this might now the deduced by simulation. The task involved six different crews, realistic flight profiles and operations, extensive human monitoring (heart rate / video).
I also participated in a type certification of an exec jet which had previously had a long and reliable history with single crew. However, later variants particularly with ‘workload reducing’ systems such as auto flight, FMS, and EFIS, were required to have two crew. The decision was based on the joint conclusion by the regulator and manufacturer (who had considered the possibility), and involved the changing operational climate as much as the aircraft type.
It would be interesting to hear more details of the task being considered; then perhaps more help might be available; my experiences are probably way out-of-date to be of further help.
There may be some helpful information in
CS AMC 25.1309 (about page 485).
Also search for information on Same Type Rating – there should be a FAA AC on this and perhaps CS policy document – see Joint Operations Evaluation Board (JOEB).
Edit
AC 23-1523 Minimum Flight Crew
Also for info
EASA - Flight Standards see OEB
http://easa.europa.eu/ws_prod/c/doc/...20document.doc
http://easa.europa.eu/ws_prod/c/doc/...B%20Report.pdf