one who seeks his horoscope comes under the ruling contained in the Prophet's statement: "The daily prayers of whoever approaches a fortune-teller and asks him about anything will not be accepted for forty days and nights." [Muslim]
The punishment in this Hadeeth is simply for approaching and asking the astrologist, even if one is in doubt about the truth of his statements. When one is in doubt about the truth or falsehood of astrological information, this entails that he is in doubt about whether or not others know the unseen and the future besides Allah. This is a form of Shirk, because Allah has clearly stated (what means): "And with Him [i.e., Allah] are the keys of the unseen; none knows them except Him." [Quran 6:59] As well as (what means): "Say (O Muhammad): 'None in the heavens and earth knows the unseen except Allah...'" [Quran: 27:65]