But can we truly see if everything is just what the brain thinks it is seeing?
It depends on what you define as "seeing" something. If you are talking about the physical process of light being directed to the retina and striking a combination of rods and/or cones which in turn trigger an electrical response via a complex biochemical cyclical reaction which then travels though the optical nerves to the cerebral cortex of the brain, then you see everything that is there assuming that your eyes are open and light reflected of whatever you are attempting to view actually reaches your cornea.
If you are considering "seeing" to being not the physical/biochemical process but the data analysis done by the cerebral cortex which forms a mental image from the numerous individual electrical impulses sent along the optic nerve from the back of the eye, then it's not really a matter of whether you are actually seeing something or not, but rather whether you have the mental acuity to extract a given detail from the overall picture.
Thus the grand, far-reaching, all-encompassing answer to your philosophical and physical inquiry you first posed is simply, yes, I like candy.
Glad I could help.