I would drain the oil and send a sample out for analysis. Go to bobistheoilguy.com for info on where to send it. They will tell you how much and what kinds of metals are in the oil. It will give a very good indication of how much damage there is.
Plus cut open the filter like you said.
Obviously if you see a lot of metal in the drain pan or filter, you may not need to send out a sample. Hopefully, there is not enough to be easily seen. In that case, I think oil analysis is about $35.
Good luck, i know that crappy feeling you get when you think you hurt your motor.