I took this still from my video at
http://youtu.be/egc09RVJ2BkLight travels in a straight line once it leaves the lens, hopefully all of it passing through a very small dot of space at the focus.
I have looked at the focus point though a usb microscope and tried to use that to focus precisely (I was after a 0.05mm dot if I could get it, photos are somewhere on the forum lol, I think in my pcb etching project thread) but found that the camera and the eyeball differ on what looks like best focus, best burning power looks like a galaxy shape, it looks like a line with a bright dot in the middle, that dot is the highest power density you are going to get. Getting that galaxy shape though is determined by lens to laser distance, the backfocus distance has an optimum as well. Moving the lens closer to the laser focuses further away.
I found that looking with eyeballs it is hard to see, the dot is so small that it does not reflect much somehow. so either side of that highly focused dot appears brighter.
What I ended up doing is aiming at a piece of paper a few feet away. Experiment with the focus to see what the spot density looked like on that paper and then move a card in between to see what focus distance I now had.
There is math that could I think be done to calculate where the focus point is, not tried it though. the lens exit is a circle of X diameter, the spot on the paper is Y diameter, distance from laser to paper is L.
Then the focus point should be X* L/Y
Unless the beam is not filling the lens aperture .. I think it does though.
If that does work then then you could draw a bunch of circles on piece of paper and write down the calculated focus distance based on the spot size, or just focus till you get the spot suze you need.