Like
Sciamano, I also take issue with the OP (original
post,
not poster 
). I think it is a
good idea to eat with the locals. And since we're talking about food and Italy, it seems to me that a little friendly debate on the topic is more than appropriate, if not downright necessary!
I think that locals do know where the good food is. And I'm pretty certain that that's where they go when they go to eat. When I dine out in Pasadena, California, I go to the restaurants that I like. I don't merely go to places that are convenient. Since
mrs. iapetus is a terrific cook, I'm not about to sacrifice quality in the name of convenience. Unless someone were to specifically request a very cheap restaurant, I would only recommend places that I think serve good food (and even then I would try to suggest cheap spots that serve quality food for the price). I can't see why Romans would be any different. In fact, I can only imagine that they'd be more so, given how passionate Italians are about what they eat.
Perche, the take-away point from your OP -- at least the point I took away -- is that visitors to Italy should be wary of assuming that the clientele in any given restaurant are locals just because they hear (or think they hear) Italian being spoken. And I think that's a very good point.
But I also think that there are other, more useful metrics to use when trying to determine if a place is truly popular with the locals. What I do is try to identify the owners of the restaurant I'm visiting. If it's small enough, I think that this isn't particularly hard to do. You then watch their interactions with the customers. If you're paying attention, I think it's pretty easy to tell when the owners know someone dining at their establishment ... even if you don't speak Italian. (I speak Italian at an intermediate level; so maybe I'm overestimating how easy it is to gauge these interactions, but I don't think so.) If I see that happening often enough, I feel pretty confident that I'm sitting at a restaurant that locals will frequent.
I also just ask people I trust for recommendations. (Not that this can't backfire. I recall going out to dinner with colleagues once at a place that had been recommended by the hotel staff. While almost everyone else loved it, a friend of mine and I were less than impressed. To have called that spot mediocre would be kind.)
And I try to avoid places with high tourist density. I recall one particularly overpriced meal with decent (not great, not bad) food right on Piazza Navona. I should have known better.
Finally, I think it's important to know what you like. I can usually tell if I'm happy with the food and service I'm getting at a restaurant. Yes, I may not be getting the most authentic Italian food. But if the food tasted good, the wine flowed and the servers were pleasant to me (bonus points if they speak to me in Italian!), I'm going to be pretty happy!