Given the system function, how can i tell if it is a lowpass or a highpass filter?
Zeroes tell which frequencies the system attenuates and poles tell which frequencies the system doesn't attenuate. If you get zeroes close to (re=1,im=0), the system doesn't allow low frequencies. If you have a pole over there it means the system just allows low frequencies to pass.  

Why the point (re=1,im=0) corresponds to low frequency????. The line connecting this point and (0,0) makes 0 angle w.r.t real axis. 0 angle is low frequency.

Similarly you can proceed for analysing higher and medium frequency range by considering suitable angle on z-plane. This just gives you a rough idea..