A tip on off-duty, plain clothes survival:
At my department, there is a policy that if you carry a weapon off duty you will need to carry cuffs to restrain a person.
If you find yourself engaged in an off-duty encounter, my department suggests pulling your handcuffs out and displaying them in a manner that can be easily seen by responding uniformed police. The idea is that a lot of loose papers, money etc. are stuffed into wallets and ID cases making an ID potentially difficult to see and confirm. A set of cuffs is easy to spot.