Descartes evil demon

Descartes in his meditations tells us that an evil demon controlls all our perceptions, but the meditator retains the power willingly to suspend judgement. The cartesian demon cannot simply force its victim to have any arbitrary chosen belief for example, it cannot install and run an arbitrarily train of thought in the mind of the philosopher, thereby making the philosopher beleave whatever the demon wants. But if this was the case in real life how would we be able to discover it and how would we deal with it? What im asking I guess is how do we know our thoughts and actions are ours to control?