1. How do I know if data from POS system are transferred through IDOC or BAPI ? Which one is recommended method by SAP ?
Ask you POSDM consultant what method they have used to send data from POS to POSDM (Usually it is POS to PI to POSDM).
SAP has given choice of sending it through BAPI, IDocs and though ES.
I will suggest you to use BAPI or ES if you POS is of third party.
2. Generally how data are transferred from POSDM to ECC and BW ? is it scheduled in batch job during night or data is keep posted during business day. I know that POS system sends data to POSDM quite frequently.