You should try training your mind's eye , the sixth chakra that is often associated with the color indigo. The Sanskrit name of this chakra, Ajna, means perception.
Daily exercises to aid in that:
Concentrate your energy at the level of your eyebrows when in a meditative state. Soon you will begin to feel pressure and tingling beginning to build in that area. Don't be alarmed.
Relax your body and mind, then close your eyes and be patient. Take note of images that arise in your mind and later dwell upon any symbolism that they might have.
These are only some daily exercises that could be done in 5 minutes. If you are serious in pursuing this, you should consult more detailed works on the matter.
Another thing you could do is begin to write observations down in a journal. Because many of us had been told that spirits/ghosts are not real, we convince ourselves that they are not real and the ability begins to get rusty. Opening our awareness to things that we consider usual and often too ordinary, will open our minds to an unordinary world.