What makes you think it might be fake?
If the EXIF data is still attached to the image, you will be able to see if the camera, lens and exposure settings. That will be enough to prove it is real.
If instead, you find it was manipulated by a program by a program like Photoshop, you will know that at least some changes have been made in the original image. Fake? That will be hard to prove or disprove unless the image was really hacked up badly
