Flipping a copy of the layer vertically and adjusting the blend mode is a good start to see the first part of a mirror effect. Maybe deform, for perspective. Liquify can help to do subtle distortion. It all depends on what you want to act as a mirror and if the perspective of the scene is a rather frontal view. A lot of times in a mirror (or mirroring objects) you see parts of the reflected object that can't be seen on the original. When that does not occur in a manipulated image, that can be a strong clue to the human eye. Good Luck!