I needed a way of pointing my solar cookers directly at the sun and came up with and made the following sun finder (sometimes called a solar finder.) I later found out that this design is commonly used for astronomy. You simply attach it to the solar cooker, or whatever you want to aim at the sun, and line it up as shown below.
How to test the alignment of a sun finder
The sun finder attached to a solar cooker
The photos below show the sun finder attached to a solar cooker. A hole was made in the front of the solar cooker to allow the sun through. The sun finder was attached to the solar cooker by simply wrapping wire around it.
Notice the two ways you can use it. You can look at the bright spot on the front of the back plate as in the first photo. Try to get the spot to disappear through the hole. Or you can look at the bright spot shining through the back plate as in the second photo. Again, line it up with the hole. A hole is used instead of having a circle drawn on the plastic in case you want to shine the spot on something else behind the sun finder, for example, a photo diode that causes an alarm to go off when the light is no longer on it meaning it's time to move the solar cooker.
Video of sun finder with full construction details, alignment testing and demonstration with solar cooker
For more sun finder, and sun aiming tricks...
For more ways on this topic see this detailed page on different tricks for lining things up with the sun.