[MonoNeuron]

I just use a very powerful magnifying glass and measure the spot size with a dial caliper. It's accurate to about 0.0005".
I place a piece of thin cardboard bent over at 1 end to form a ramp about 10mm high and 50mm long then burn a spot every 5mm up the ramp. Then all I do is find out which spot is the smallest and measure that against the height from the lens to also get the focal distance for setting up the laser head.
regards
Richard.