I don’t think you can know. Last year, I had interviews that I thought went poorly. My first interview weekend I told my parents that I didn’t think I got in because one of my interviews turned into a lecture on terminology that I had used correctly. I got in. Then I had another interview where I felt I was in a corner and stuck and found out later from their student that the professor liked me and thought I did well. I did not get in. On the converse, I had interviews I thought went well and then got waitlisted or outright rejected.

I think you should do a little reflection on how the interview went and where you got stuck/tripped up or didn’t anticipate something. Take that data to improve. But then don’t worry about it. Do your best in every interview and know that you did your best so even if you didn’t get in you gave it your all. You get in or not. You can’t say what part(s) of your application/process worked/didn’t work. I didn’t get any feedback on my interviews except that I got in/not.

Yes, a thank you is good practice. But just keep it as a thank you.