Diagnosing a suspected multi wire branch circuit
Years ago handle ties were not required. They have been for decades , if you think it’s is a multi wire turn off both breakers verify both hots in your box are dead, you can open your breaker panel and most of the time NMB or Romex is used. There will be a red on 1 breaker a black on the other going to a cable with a ground and a neutral, that would be 100% verification.