Maggie put her finger on it with one of her comments on James Van Praagh, there's no way of knowing (for sure). There's lots of debate over psychic abilities and whether they exist and can be measured. Like many on the forum, I believe they do exist, indeed my wife gives me living proof of that, and to a limited extent I feell I have them too.
Within a natal (or other chart) there's no agreed signatures. Neptune would be an obvious candidate, given the psychological view of the planet but how it would identify as such is anybody's guess and fundamentally unprovable. There's also the mild issue of whether psychic abilities are genetic or learned, in which case the natal chart may be of limited, or no use.
We can dismiss Neptune's sign, as it is in each sign on average for around 15 years - we can't say that ALL the people born during that 15 years will be psychic and NO ONE will be born psychic for another 160 years.
Neptune's house placement would be something to consider but we would need to decide on which house reflected psychic abilities. The link with the twelfth is a very modern view, and is perhaps a reflection of attempts to give the twelfth a better image. The house of spirituality is the ninth, which is also the house of long distance communication and travel. It is the house of unusual or non-routine communication. So I'd favour that but, I repeat, there's no way of testing this.
Aspects to Neptune are the next possibility, I'd look at very tight conjunctions, or trines to Sun, Moon or Mercury, or possibly the Ascendant or MC.
However, all of that presumes that Neptune is involved somewhere, rather than Uranus (the unusual) or Jupiter (ruler of religions and spirituality), or, of course, the Sun and Moon.
Most Astrological explanations are constructed to suit a particular view of the psychic, and until we have an accepted recognition and definition of what 'psychic' is we will be left speculating.
