The extra cost of long lenses goes into bigger glass, not sharper images. Things to look for are sharpness and contrast, as well as lack of flare and even illumination. To test for even illumination, take a shot of the clear blue sky. Check the image quality in the corners of the frame too. Most lenses are significantly worse in the coners than in the center, so look carefully there.
Lenses also differ in color balance.
In the end, only you can decide if the lens meets your needs.