if it is a single PIR detector
( Passive infrared sensor - Wikipedia )
u cant...
some of them have 2 elements inside, so that u can tell if the object moves from right to left (first high peak then low peak) or from left to right...
if u need the position u could use 2 cameras...
but the arduino cant do the computations in reasonable time, i guess...
a grid of thin angle PIR sensors might work... like this:
v v v v