There is no way you can tell if an RCA connector is 75 ohms unless it is marked that way. In fact, the only true 75 ohm RCAs that I'm aware of that can be easily purchased are made by Canare, and they are clearly marketed as having a 75 ohm characteristic impedance.

There is a way to tell of a BNC connector is a true 75 ohm, but no easy way to describe the difference because it is subtle.

Honestly though, there is little degredation that will likely take place if just the connector is not a true 75 ohms and the rest of the cable is.