how to validate data while idoc is triggering ?

Hi

1) I have activated message type in BD50 tcode (HRMD_A is checked ) .

2) Done changes in PA30 tcode.

3) Executed tcode BD21 with message type HRMD_A then I observed idoc are triggered.

Now my questions

1) if I want to validate data then where exactly I can validate .

2) if I have custom fields how can I populate that data into IDOC.? how can i make those fields are change pointers ?

Appreciate earlier response..

Regards,

SRK