Hi,
As mentioned by other members, you should use GETFLDSTATE() and GETNEXTMODIFIED() to detect changes on your own local buffered data. Then,

FOR I = 1 TO FCOUNT()
  lcField = FIELD(I)
  IF CURVAL(lcField) # OLDVAL(lcField)
    *--Conflict
    RETURN <ERROR>  
  ENDIF
NEXT

*-- No conflict, issue TABLEUPDATE()
IF NOT TABLEUPDATE(.T., .F.)
  AERROR(laError)
  DO CASE
  CASE laError[1] = 1585
     RETURN "CONFLICT"
  OTHERWISE
     RETURN "ERROR"
  ENDCASE
ELSE
  RETURN "OK"
ENDIF

IF, your meant is how to know your conflict handler routine works correctly? You can have two instances of VFP running at the same time in same machine, Locate same record and make changes in both instance. Then, issue TABLEUPDATE() on 1st instance, after that go to 2nd instance and issue TABLEUPDATE() again. You will found that TABLEUPDATE() return .F. with error code 1585 in 2nd instance.

HTH