In addition to professional reviewers/bloggers, take a look at 2spaghi.it. There's no English version, so the traffic and reviewers are going to be a different demographic than what you find on TripAdvisor or Yelp - even on the Italian versions of those sites. There are generally fewer reviews, but for my tastes the accuracy is good. You can get by with Google Translate if you need to, but it might also encourage you to learn a few words before you go

For what you didn't plan in advance, what works up here in Maine works in Italian cities too - ask people. The best meals I've had near home were found the same way I found the best meals I had in Italy or anywhere else - word of mouth.

Reservations are a good idea. I did a lot of mine same day this past February, or a day out, but traveling with a toddler I also wasn't looking for 3 michelin stars.

I've said it, Perche has said it, but don't rely on your concierge - or anyone else in the tourism industry - for recommendations. Odds are high you're getting sent to their friends, also in the industry, and the circlejerk is complete. I might make one exception and that is if you take a food tour, the guide might have good recommendations because they're not going to risk their reputation and bad online reviews by sending you to eat crap. The other exception is asking people who work at a good restaurant about other good restaurants - if I enjoy a meal, I'll often ask where they eat or where they'd send their friends.