cOldError = ON("ERROR")
ON ERROR *
USE MyTable IN 0 EXCLUSIVE
lSuccess = used("MyTable")
ON ERROR &cOldError

if lSuccess ...

nFHdl = FOPEN("myfile.dbf", 1)  &&  1 tries to open the file for writing
IF nFHdl > 0 THEN
   FCLOSE(nFHdl)
   USE myfile.dbf exclusive
ELSE
   = messagebox("Can't open Exclusive")
ENDIF