First you want to learn how to run a trace. Check out this thread: How do you read/analyze a trace log?
Once you get a handle on running the trace, you want an easy way to parse through the data. Check out Jose’s Trace Helper Utility. Trace Helper Utility for Epicor ERP 10
Once you have those in place, run a trace while creating the labor detail the normal way (through time and expense entry, I think). The trace will log all the methods that epicor uses to create the records. Often it logs waaaaaaay more than you need. So you will have to step through the trace with that utility and look for the places where labordtl records are created. This take a little bit of deciphering, but its not too difficult. I imagine that your method will look something like this: Erp.Proxy.BO.LaborImpl.GetNewLaborDtlWithHdr()
Good luck!
Nate