Eloy -
Our technician's go through OJT as well for calibration work. When they are ready to be approved for a specific Calibration Method, the supervisor submits a sample of work they performed and a written statement from another technician who has worked with them on the cal method to QA. If it is satisfactory, it is forwarded by QA to the Technical Manager who has the final approval.
If he approvals the technician, the Technical Manager initials & dates the Calibration Method on the Proficiency Document which is maintained as evidence of competency for the individual technician. There is also a section to cover use of equipment, such as Load Cells, Temperature Monitoring, Volt Meters, etc. If applicable to the approved method, that section is also initialed and dated.
It makes a nice tidy packet to show as objective evidence for training.