A Sikh should not believe in star signs. Though I can and will support this statement, I am certain the discussion will continue if the message is not what some wish to hear.
It is actually beadbi to seek the advice of astrologers, or consider horoscopes.
From the Sikh Rehat Mayrada,
(d) Not believing in cast or descent, untouched ability, magic, spells, incantation, omens, auspicious times, days and occasions, influence of start, horoscopic dispositions, shradh (ritual serving of food to priests for the salvation of ancestors on appointed days as per the lunar calendar), ancestor worship, khiah (ritual serving of food to priests - Brahmins - on the lunar anniversaries of the death of an ancestor), pind (offering of funeral barley cakes to the deceased’s relatives), patal (ritual donation of food in the belief that that would satisfy the hunger of the departed soul), diva (the ceremony of keeping an oil lamp lit for 360 days after the death, in the belief that that lights the path of the deceased), ritual funeral acts, hom (lighting of ritual fire and pouring intermittently clarified butter, food grains etc. into it for propitiating gods for the fulfillment of a purpose) jag (religious ceremony involving presentation of oblations), tarpan (libation), sikha-sut (keeping a tuft of hair on the head and wearing thread), bhadan (shaving of head on the death of a parent), fasting on new or full moon or other days, wearing of frontal marks on the forehead, wearing thread, wearing of a necklace of the pieces of tulsi stalk [A plant with medicinal properties], veneration of any graves, of monuments erected to honour the memory of a deceased person or of cremation sites, idolatry and such like superstitious observances. [Most, though not all rituals and ritual or religious observances listed in this clause are Hindu rituals and observances. The reason is that the old rituals and practices, continued to be observed by large numbers of Sikhs even after their conversion from their old to the new faith and a large bulk of the Sikh novices were Hindu converts. Another reason for this phenomenon was the strangle-hold of the Brahmin priest on Hindus secular and religious life which the Brahmin priest managed to maintain even on those leaving the Hindu religious fold, by his astute mental dexterity and rare capacity for compromise. That the Sikh novitiates include a sizable number of Muslims is shown by inclusion in this clause of the taboos as to the sanctity of graves, shirni, etc.]
Not owning up or regarding as hallowed any place other than the Guru’s place - such, for instance, as sacred spots or places of pilgrimage of other faiths.
Not believing in or according any authority to Muslim seers, Brahmins holiness, soothsayers, clairvoyants, oracles, promise of an offering on the fulfillment of a wish, offering of sweet loaves or rice pudding at graves on fulfillment of wishes, the Vedas, the Shastras, the Gayatri (Hindu scriptural prayer unto the sun), the Gita, the Quran, the Bible, etc.. However, the study of the books of other faiths for general self-education is admissible.