Is there a good way to test your schematic before buying the parts and building it?
No there is not, computers will not be able to do the jobs of electrical engineers for a long time, if ever. 

The best bet is to simulate the parts of the circuit that are most complex in a spice package. I would test the DC to DC in a spice package. If your really unsure, I would test only one of the DC to DC converters before building many of them. 

Most schematic designers also have Design Rule Check (DRC) packages that you can check voltage compatibility, input/output compatibility, ect. to help the designer spot mistakes. This also takes a lot of time to setup. 

A few other basic tips:  


Read the datasheet through for all new parts that don't have history (ie the circuits that are new to you)
Check each part for bypass caps
Check the pinout for all connectors and make sure they match the cables.
Check every pin voltage.  
Make sure there is enough current/power for all the parts
Check programming interface pinouts