In some cities, there's either no organised walking tours, or just one, so there's nothing to choose. Sometimes, you want a specific tour, perhaps architectural or to see movie/TV filming locations, and again there's few/no options.
Other times, you have a surfeit of choice, with several different free and paid-for tour operators running a general 3-ish hour "sights of the city" tour. In such well-served cities, you might have 4 or 5 options for a tour leaving mid morning or early afternoon covering the main sights to pick between!
What approaches / techniques / strategies can you use to investigate the options, to work out what's likely to be the best tour for your personal likes and situation, when you find yourself with a choice like this?