How can I tell if I have a 170 or 225 s6 in my 64 Dart?

I have an AZ 64 Dart 170 I just picked up and I was wondering how I can tell which engine it has.

The guy I picked it up from was not a car guy so I don't know if becaause it was a 170 car he may have assumed that it had that engine. I haven't driven it yet so I don't how well it performs. Runs smooth though.

Thanks,
Mike