Zodiac P.I. Vol 1

From the Tokyopop edition

Riri is just like any other normal girl - except she can foretell the future! This "Zodiac P.I." transforms herself into the magic Detective Spica and looks to the stars to guide her way. Before long, she is joined by her childhood friend, Hiromi, who dreams of becoming a detective as skilled as Riri. Soon, no astral investigation is too arduous for this dynamic duo!

Source: Anime News Network Encyclopedia