SpaceX has the Dragon 1 (Cargo) and Dragon 2 (Crew). It unveiled Dragon 2 about 1 year ago (30 May 2014). Dragon 2 is a radical change over Dragon 1 with many parts that are untested.
I would think that the best way to test Dragon 2 for human flight would be to start using it for the cargo missions. There are a lot of aspects about Dragon 2 that could be tested in cargo missions that would be useful to ensure that the design is optimal. Some things I would think could be tested include:
- Aerodynamics and heat shield for re-entry
- Parachute landings
- Electronics, software and other sub-systems
- Docking mechanism to ISS
- SuperDraco engines in space conditions
- Life support systems
- General reliability of all the components used
SpaceX seem to prefer testing more and using existing missions to test. Since they are already doing the resupply missions, why not test the Dragon 2 on those missions?