how do i no if my bullmastiff puppy is worth showing ?

Only by letting somwone expereinced in the breed assess her when she is older.  Until she is mature no-one can say if she will turn out well for sure.  So the only way to know is to take part :D  It is the one place no=one will mind you being obsessed by dogs :D  All us dog mad folk know well that glazed lok in the eyes of our non dog mad freinds and family :D