Watch how the whole timing thing works:
http://www.animatedengines.com/otto.html
First and
after eliminating all metal debris from the engine, I would check if timing is as you last adjusted it.
I would be more worried about the two valves which rockers didn't break off.
The proper way to verify damage is dissembling the top and sending it to a machine shop.
Without going that expensive route, you could check a few things:
1) If the stems of the valves have been bent: Visually check if any valve is not returning all the way back: bad news if so.
2) If #1 or deformed valve lip or broken piston: Run a leak test with all the valves of that cylinder in closed position.
http://www.dansmc.com/leakdown.htm