What kind of sensor can detect when two things are pointing at each other?

Using Arduino Sensors
1

I need some way to detect when two things are pointing at each other. I am using an Arduino Uno and would prefer the easiest way of doing this. The precision has to be very very precise though. Thanks in advance.

2

Hi,
How about a simple laser and phototransistor or Photoresistor on one unit and a simple mirror on the second unit?

The first unit would have the laser and the detector close to each other pointing out.

Using a simple visible laser like this would allow you to see exactly where the light beam goes. Like:
http://yourduino.com/sunshop2/index.php?l=product_detail&p=404

This would be VERY precise.

DISCLAIMER: Mentioned stuff from my own shop...