how can validate tablecontrol fields while inserting records?

hi, all,

my requirement in table control iam entering some records in that some fields are not in format or already exist in database .so when i press enter i should display the error message and cursor is placed where the record field is from starting .If I correct and press enter cusor is placed next error automatically

so how can achieve this please give me any suggestion and any samplecode

satheesh.