A cat scan without contrast can that still detect a brain tumor ?

Yes: Contrast dye helps to show the vascularity and fluid areas however a normal ct will show tumors and areas of infection.

Not always: It is much harder to detect smaller lesions on non-contrast head ct, but large tumors causing swelling or pressure on other parts of the brain can usually be detected. Usually brain MRI with contrast is more sensitive and gives me a more detailed picture when I am trying to evaluate for brain tumors.