>in multiuser environment how do i detect, that the file is open by someone

You could do something like this.

ON ERROR CatchExclusiveError() && special error handler to catch if table could be used exclusively
USE mytable EXCLUSIVE 
ON ERROR MyNormalErrorHandler() && enable normal error handler

PROCEDURE CatchExclusiveError()
	IF ERROR() = 1705
                * table couldn't be opened exclusively. Someone has it open
		* do processing or actions if table could not be opened exclusively
	ELSE
		* do processing or actions if table could be opened exclusively
	ENDIF
	RETURN
ENDPROC