How do you spot a tourist trap?
By recognising that the signals are consistent and mostly about how a place acquires customers — a business relying on one-off visitors who will never return behaves differently from one relying on people who come back.
The structural signal. A restaurant whose customers are all first-time visitors has no incentive to be good, only to be found. A place relying on regulars must be good, because its customers have alternatives and memories. Almost every practical indicator follows from this.
The recognisable signs:
Staff outside soliciting customers. A place with a queue does not need someone in the street persuading you.
Menus in many languages with photographs, positioned to be read by people walking past. It signals an expectation that customers cannot read the local language and will not return.
Immediately adjacent to a major attraction, where footfall is guaranteed regardless of quality. Distance from the landmark is the single most reliable variable — two or three streets back frequently transforms both price and quality.
A very long menu covering many cuisines, which nothing can execute well.
No local customers, which is the clearest test if you can observe it at a meal time.
Prices without portion sizes, particularly for fish and meat sold by weight, which is the mechanism behind the classic inflated-bill dispute.
Aggressive "free" additions — bread, sides, entertainment — that appear on the bill.
What actually works instead:
Eat at local times. Tourist-oriented places serve continuously; places serving locals fill at local meal times and are empty otherwise, which makes the distinction visible.
Walk away from pedestrianised tourist streets, in any direction.
Look for a short, seasonal menu, and handwritten changes.
Ask someone who lives there and is not employed in hospitality at your hotel.
Read reviews for the complaints, not the ratings, and weight recent local-language reviews above tourist ones.
The fair caveat: a place being popular with tourists does not make it bad. Some genuinely excellent and famous establishments are full of visitors for good reason, and reflexive avoidance of anything popular is its own error.