Hi Chris,
Just to clarify, are you talking about Adapters (Erp.adapters.JobPart.dll)? or BOs (Erp.contracts.BO.JobPart.dll)?
For “client” BOs, there is technical documentation provided by Epicor for both, e9 and e10, listing all BOs and their method signatures plus a brief description of what they are for.
For Epicor 10 its under documentation:
Go to E10 documentation
And download :
For Epicor 9 its a bit different, its not on documentation, but on downloads:
Then on the right panel download:
Now you get the same doc, but on a pdf:
That’s the theoretical part of your request. if however, you are not satisfied by the some times insufficient short description provided, then you should move forward to the next level of research 
Go to your Epicor ERP directory and find the BL Tester tool. It should be located under ERP10 > updates folder. Configure it to connect to your e10 TEST or DEV environment, switch company/user if needed, then load the Erp.Contracts.BO.JobPart.dll. You will get the list of methods and you can easily test them.
Enter parameters and click on invoke.
You will get the results once you invoke the method:
A more technical approach is to use tools such as ILSpy, JustDecompile, dotPeek or Reflector to look at the disassembled code of the “client BO/Adapter” (..\ERP10\LocalClients\ENV\Erp.Contracts.BO..dll) or “server” BO assembly (..\server\assemblies\Erp.Services.BO..dll) and try to figure it out what the method really does.
Regards,
Carlos