TripAdvisor, and although it has both perks and flaws, the reviews are unavoidable for most restaurants.
The key to any restaurant, big or small, is the quality of food you serve. If the food you serve is a WOW than you are definitely going to get a lot of WOM (word of mouth) marketing - this is the best form of business.
I am not sure sustainable food is the only deciding factor for diners to choose a restaurant. I'm writing this based on many popular tourist destinations in SEA and hugely populated countries like Indonesia, China, India & Bangladesh who are unable to implement this in their restaurants.
The hospitality industry, especially 4 & 5-star hotels, spends lots of resources on training the staff.