I don't believe that any kind of vetting or questioning is effective.
My strategy is to start small -- with a contract for no more than an hour or two of work, to give the other party an opportunity to demonstrate how they will behave. If everything goes as expected, then increasingly large milestones/contracts can be agreed to.
If the other party does not treat the fixed-price contract model as you expect them to, then you aren't into something so big and time-consuming. If necessary, you can simply walk away, closing the contract and leaving the funds to them.
