How do you know where your feet should be when squatting?
A good place to start is with the feet shoulder width and the toes slightly pointed out. However, each individual lifter will have to experiment with this in order to determine the optimal stance for squatting.
Utilize your strengths…
Individuals with stronger quadriceps will tend to squat with a narrower stance and with the toes pointed more straight ahead in order to take advantage of the greater quad activation with this stance.
On the other hand, those with stronger glutes and hips often prefer a wider stance with the toes pointed out in order to take advantage of greater activation of the glutes and the muscles surrounding the hips.
Another consideration is technical proficiency. An individual that competes in strength sports will want a squat stance that allows them to squat to a depth of just below femur parallel, while keeping the chest upright with a neutral spine, heels flat, knees in line with the mid-foot, and shins at or close to perpendicular to the ground. It may take some experimentation to determine which squat stance is best for you.