Can a ct scan with contrast of the stomach detect stomach cancer and how early?

Sometimes: Generally an upper endoscopy can be more sensitive.

Sometimes: A ct scan with contrast can 'see' a tumor in the stomach wall if the tumor is large enough. Small tumors, however, can be missed because they do not cause stomach wall thickening. An upper endoscopy, a procedure where a physician looks directly into the stomach with a scope/camera is much better at detecting stomach cancer.