When you are outside of your normal work environment, and you come across someone who needs CPR, generally speaking you are protected by the Good Samaritan laws of your state. What that means is that unless you know for sure that someone is a DNR/No Code, you can start CPR and you will generally be protected against lawsuit, even civil suit, unless you do something that is grossly negligent. The only thing that I would add to that is not to exceed the care given by someone trained in first aid. The laws/regulations of your state may allow a nurse or other advanced level provider to provide some advanced level care, most probably will not. If you exceed that level of care that they authorize, you will be at risk for lawsuit. From my understanding if you provide care at a level no greater than first aid, you will be fine. That does not mean that you cannot assess the situation at the level of training you have, but, the care you give should not be any higher than that of basic, or even advanced first aid.
When I am working as a paramedic, if I go off duty and I come across someone in need of help, I am safest in providing care at a level no greater than that of the basic EMT. That does not mean that I cannot assess at a paramedic level, rather I must (and in some cases required to) provide care at the EMT level. With nurses, the basic idea is the same. You have a lot of knowledge and experience in providing rather high level care, but, when you are not working as a nurse you must restrict your care to that of essentially an EMT. That is, for all intents and purposes, advanced level first aid. Stick to that guideline and you will probably never have to worry about seriously being sued. Yes, someone could try to sue you, but the Good Samaritan laws in your state should provide protection against that lawsuit.