score:4
The obvious initial answer for filtering your selection is to ask them where they go. Some may be offering an 'historic' tour, a 'river walk', 'museum stroll' or whatever - and depending on the variations of routes, even for a 'sights of the city' tour you may see some variation and see something one has that others don't that you don't want to miss.
Once you've narrowed it to the set that matches what you want, look at who they are. Many these days are free - run by students, or as a loss-leader by companies who then promote their paid tours at the end. Consider if this affects your decision.
Next, if you're going with one of the companies that also has paid tours, look at their paid tours and see if any look interesting to you. If so, it would be worth going with their free one, so that you get an idea for the company's pace, style and information.
Finally, one of the big sellers for me in Berlin was asking the tour guide's history. They were a British student doing their masters in European History, and they lived and breathed this stuff. The perfect informative person for such a tour.