How to know if Orlando club shooting was an act of Muslim extremist terrorism?

The terror is still under investigation, but what are some indicators that this was an act of terrorism by an extremist terrorist organization?

The killer was of Afghan origin, but nothing so far indicates that it was Islamist extremist. Not every Muslim is an extremist, any more than every Christian is (a concept which seems difficult for some people to understand, including some presidential candidates).

It seems pretty clear he did this because he was a extreme Muslim, but law enforcement is still investigating if there was an organization directly involved