How and where to validate datefield value on form.

Hi, I have scenario where i have 2 datasources on single form Some controls has datsource = DS1 Grid das datasource = DS2 when user enter enters values for controls for datasource1, how/where can i validate whether user has entered values for controls/grid for 2nd datsource as well. DS1 and DS2 has relation. How and where (which method) can be used to validate same? Thanks, Ragards, Rahul$