How do I find the Field name to use in Meta Queries?

Go to the field you want to use in a Meta Query, click in the field and hit the F1 key. This will pull up the Field Help box showing the Field Name and the name of the file. 

Example: On the Personal Data (KE0039) screen, click in the field for Birth Date and hit the F1 key. The name of the field for Birth Date is BIRTH.DATE. The name of the file for Birth Date is EMPMAST.

Hint: If the field you want to query from is greyed out, click in another field and hit the F1 key, then select Screen Help, then Screen Flow. That will bring you to a report which shows all fields on that screen.