You know, to determine if the cause or sponsoring organization is one you support.
You don't really need to interrogate the kids standing by the street and waving signs. A simple "What are you raising money for?" should suffice.
Most times, a car wash is just a car wash. The donations support band trips, someone in need or whatever.
But it's better to ask first and not find yourself realizing after it's too late that the sudsy event is a fund-raiser for a group, church, project or club whose aims you either oppose or regard as damn foolishness.