1.  If you are looking for an ethnic restaurant visit the restaurant and check the patrons. Is anyone of that ethnicity dining there?  If yes that is  a sign that the food is most likely authentic.
  2. Does it appear to be locals or tourists in the restaurant?  If it is nothing but tourists you would be taking your chances.  If it is locals then it is a good bet that it has a favorable reputation in the area.
  3. Do they take reservations, why or why not?  If no is it because they are never busy enough to need them, or are they too booked to take anymore?  If it is the later ask if you may wait list and stop by towards closing when you may be able to get a table that has just opened up.
  4. Check the table setting.  If there are flowers are they fresh?  Are the candles burned down too low?  Is the table setting complete with clean table cloth and napkins?  Attention to detail would indicate a well run restaurant and a well run kitchen as well.
  5. Sign boards with a menu printed are ok but a barker standing next to it pointing out the specials is not. This would most likely be a tourist trap with less than average food.

One could even go as far as checking for burned out lightbulbs, finger marks on the entry or exit and certainly clean bathrooms.  I am a stickler for a well kept eating establishment.  Use the top 5 as a model and enjoy your meal.