Now on to some other, less cut-and-dry tips. These aren't "all or nothing." Think of them more as building blocks to an even greater possibility of having a delicious local experience. These can get a little ambiguous, so take everything below with a grain of salt as you explore your own foreign cities!
1. Keep your eye out for locals. Considering we want to eat where the locals eat, it's always a good sign if there is a smattering of locals in the restaurant! An empty restaurant isn't usually a good sign. (Although beware of local dinner times. It's possible that you are just missing the local crowd by eating at an odd time.)
2. Look for a hand-written menu. A hand-written menu could mean that the chef changes the menu so much that it wouldn't make sense to print out a new menu every other week. Chefs that change menus put more effort into their work and often have a greater variety and passion for the food they cook. Plus, it's less probable they buy prepared food if they change their menu that often.
3. Pay attention to the interior. Cleanliness is huge (obviously) and a dirty storefront or seating area probably signals a dirty kitchen. Yuck. But past that, cooking is an art, (hence the term "culinary arts") so if you walk in to a restaurant whose decor doesn't seem very thought out or intentional, it's possible the food isn't very thought out either. A restaurant with a passion for food will want to compliment their dishes with a positive atmosphere. I'm in no way saying it needs to look expensive—in fact often the best restaurants don't—but a restaurant with intentional interior decor can say a lot about the priorities of the people who run it.
4. Figure out a reasonable price for the most common local appetizer based on local restaurants you have found. Then, whenever you are checking out a new menu for how reasonable the food is, check how much they are charging for that appetizer to gauge how their prices compare! This is something I have begun to do around Spain (for me here in Barcelona, that would be the delectable "patatas bravas").
Finding a great local restaurant often takes time wandering through little streets peeking in countless little restaurants until you find the perfect one. Take this time wandering as a wonderful opportunity to explore the less touristy areas of the city you are in. Good luck and buen provecho!