I think the important thing to remember is that, if a situation has gotten to the point where you're contemplating helping out an LEO, it's a a high-stress situation for the cop who's involved and his decision-making skills--including his ability to discern between friend or foe--may be compromised.
Absolutely the first thing you should do is call 9-1-1. Tell them what's going on and, if you're going to attempt to intervene, let them know that, giving a description of yourself. Keep the line open and then ask the cop on the scene if he needs any assistance. Odds are that he'll want you to keep out of it, unless things are going south in a serious way.
As far as employing your concealed weapon, I'd say that the only time--absolutely the ONLY time--to pull it out and use it is if you feel the good guy(s) are in imminent danger of serious harm or death. Doing it at any other time complicates the cop's life and places you in danger. It may also compromise the officer's safety, as his attention may then be split.
I was an LEO for a few years and I can't think of any situation in which I wish an outsider would have gotten involved in a physical confrontation or apprehension. On the other hand, there were times when I would have been more comfortable with backup, but the situation was evolving quickly enough that getting on the radio at the moment was not practical. If you see a cop in trouble, your first reaction should be to call 9-1-1.
There are a lot of threads that are started here that seem to be based on the unspoken premise that anyone who carries legally is a Junior Deputy. Not so much. Unless an LEO specifically requests your help, you have no special legal status and the legal and practical constraints that apply in any other situation still govern.