Distributors are true businessmen. In the end they're the ones to quote the figure & buy the film.
You're forgetting the point that they're not amateurs. Some of them have been in business for decades & even have it as family business.
You show them few clips most of them will instantly figure out the fate of the film. It's only in the case of huge superstar's films they fumble or sometimes with smaller films which they over / under estimate. For example, Sardaar Gabbar Singh was sold even before film was shot. Infact they were still shooting song a week before the release & got the song censored by the cbfc separately. This doesn't happen often.
That's why I always say the best critic of any film "quality & business wise" are the distributors as well as theatre personnel (who've worked for decades), not the newbie's who work part time now.