How do you report reckless pilots?
The FAA provides an online reporting tool here I would say it falls under "aviation concern". In your description if you include tail number and time they should be able to figure out who was flying (or in the cockpit if its a 2 man situation). You can also contact your local FSDO and report the tail number and time of incident. Aircraft owners/renters (the people the tail number will trace back to) will more than likely know who was flying a given tail number at a certain time so as long as you know when it happened you should be ok its not necessarily on you to know who was flying but the tail number and incident time is key.