The frequency of light emitted from a source is changed. What visible evidence would indicate this?

Answer: If the light is visible, you would notice a change in the colour of the light. The frequency of a light ray is relationed with the wavelength in the next way λ = v/f.

where λ is the wavelength, v is the speed of the light (in this case is c) and f is the frequency.

The visible wavelength are in the range of 380 nm to 750 nm

where 380 nm is a blue extreme, and the 750 nm is the "red" extreme.

Then if your frequency decreases, your wavelength increases and the light turns more reddish, if your frequency increases, your wavelength decreases and the light turns more blueish.

The change of color. Each color has a different wavelength therefore a different frequency.