[erlang-questions] Test if function has been called

[Constantine Povietkin]

Hi all,

I know my question has been discussed _a lot of_ times, but I've no
luck to find an answer.

I've a gen_server based module that should call handlers with apply.
Method that calls handlers should not return anything by design.

What is(are) way(s) to test if function has been called.

Thanks in advance!

P.S. Links to tools/documentation are highly appreciated.