I have what feels like a callus on the bottom of my toe and it's very painful. how can i tell if it's a corn or a tumor?

More likely a corn: Very rarely would you have a tumor on the bottom of your foot. Most likely you are dealing with a corn or IPK (Intractable Plantar Keratosis) or a plantar wart. Both can be very painful with pressure. Warts have tiny little black dots and no skin lines running through them. Corns have no black dots and have skin lines running through them. A tumor would have to be biopsied and sent to a lab.