With a LASER pointer
This is by far the most accurate method that I have yet discovered.
Many of us have such a pointer that is used for pointing at details of an overhead presentation during a conference and before a large audience.
The low powered units cost around 20 to 30 US Dollars and can be bought at an Office Depot store for example.
The LASER power of a LASER pointer is small enough as not to damage the CCD sensor of a camera or its electronics. As a matter of fact its light flux density is much smaller than the SUN shining and burning through your front lens. However this author is not responsible for possible subsequent consequences of the repetition of this experiment by anyone else.