How can I identificate an unit parameter ?

You can look in the unit AST but the simplest method is to test if the INSTATUS or OUTSTATUS attributes return a value different from FALSE.

SCRIPT testparameter() 
  IF ($cunit.SYMBOL("x").INSTATUS) THEN
    PRINT "The variable x is a parameter"
  ELSE
    PRINT "The variable x is not a parameter"
  ENDIF
ENDSCRIPT