As long as they are isolated, their state is a superposition of all possible states, and evolves determinsitically, with the amplitude of each of these "sub-states" evolving perfectly determinsitically. If you want to perform a measurement, you choose a possible decomposition of the superposition state and measure along that axis, and you'll get one of the values along that axis, with a probability that is the modulus of the square of the (complex) amplitude of that value.