[math-fun] Random binary experiment

You have to perform a random binary experiment in front of a crowd of people — so no fooling is allowed — in such a way that everyone is convinced that the experiment was fair. The people include some technical experts but many who are not. What is the simplest / easiest / cheapest way to ensure that the crowd will be convinced that the experiment was fair (the two outcomes had an equal chance of occurring) ??? —Dan P.S. I do not have an answer to this, but maybe there is a "best" answer.