Before we can ask a person why they think their statement is true, we have to understand what they are trying to say. If a statement is unclear, a critical thinker will first seek to clarify the statement before challenging it. It is quite common for people to use ambiguous or vague language in stating their beliefs. For example, suppose someone says "Summer school is too expensive." It's impossible to agree or disagree with this because it is not clear. A critical thinker will notice that it is unclear and before doing anything else, will stop and ask questions of clarification so that she can respond intelligently.
Suppose that you hear this and see two possible things it might mean. You might then ask "Do you mean that it costs more than you can afford right now or do you mean that the expense is unjustifiably high?" Offering the person options like this is usually the best way to clear up any confusion.
But sometimes you simply have no idea of exactly what they mean. In which case you just have to ask directly. For example, if someone said "Love is political," I simply wouldn't know what they meant. I would have to ask "In what way is love like politics?"