I had a vendor ship me a 2" lens, turned out it was actually a 1.5"
Thought I was going mad trying to get the thing to cut or engrave, I did refine a whole bunch of other things on the machine as a side benefit
Take the lens out and position yourself under an overhead light with a ruler. Hold the ruler vertical, resting the end of it on a surface. Then hold the lens perpendicular to the over head light and beside the ruler. Center the focused image as best you can that appears on the surface and then move it up and down, it will be pretty obvious when it is roughly in focus, check where the bottom of the lens is according to the ruler.
As these lenses are normally in 1/2" increments of focus it’s pretty easy to see where it lands. This confirmed for me what I had suspected with the lens I was shipped. My focus area was up inside the nozzle with the 1.5" one installed