How to check if BAdI is implemented?

There are several possibilities to do that however almost none of them is really 100% check. I will go through them and will see that you can get out of them.

Enhancement concept tables/views:

V_EXT_IMP - View of all BAdI implementations (check field ACTIVE = 'X')

V_EXT_ACT – View on active implementations for an exit 

BADIIMPL_ENH - Implementation Active/Inactive Runtime Table (check field ACTIVE = 'X')

BADIIMPLENHCUST - Implementation Active/Inactive Runtime Table (check field ACTIVE_IMG = 'X')

TA SE84: this is probably the best way

Go via Repository Information System -> Enhancements=>

Classic BAdIs (Impl.)

Enhancement Implementations

          Dictionary Enhancement Impl.

Composite Enh. Implementation

and then enter Z* or Y* for customer implementations name field