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.
ਜੇਨ ਕਲਾ ਧਾਰਿਓ ਆਕਾਸੰ ਬੈਸੰਤਰੰ ਕਾਸਟ ਬੇਸਟੰ ॥
Jaen Kalaa Dhhaariou Aakaasan Baisantharan Kaasatt Baesattan || His Power supports the sky, and locks fire within wood.
ਜੇਨ ਕਲਾ ਸਸਿ ਸੂਰ ਨਖ੍ਯ੍ਯਤ੍ਰ ਜੋਤ੍ਯ੍ਯਿੰ ਸਾਸੰ ਸਰੀਰ ਧਾਰਣੰ ॥
Jaen Kalaa Sas Soor Nakhyathr Jothiyan Saasan Sareer Dhhaaranan || His Power supports the moon, the sun and the stars, and infuses light and breath into the body.
Kartar Purakh supports the universe and all that happens within creation , accrording to Divine Hukam. This frees us from submission to superstitious beliefs in which other humans, like astrologers, have the upper hand in our lives, and collect money to control our affairs..
I remember a Physics lessons when I was at school where we did a calculation of the force exerted on a newborn baby by the moon and a table in the room. The table exerted a larger force on the child so thinking that the moon or planets can have any effect on someone's life is hilarious! If you read horoscopes they are often very vague and can be twisted to suit anyone. There is also the possibility that if you have the horoscope before hand you will act accordingly to make the things come true. This can be consciously or subconsciously.
Another interesting thing I read somewhere (will try and source it) said that astology is no longer relevant anymore anyway even if it was before as the paths of the planets has changed significantly so the alignments no longer work. There is also the issue that technically there should be 13 star signs not 12. I can't remember why but remember the explanation seemed plausible at the time I read it. I see star signs as entertainment, nothing more.