With most(?) arcade monitors, the focus pot is located on the flyback transformer, along with the screen pot that controls the voltage to the electron guns. If you remove the outside case of your PVM and look in from the left (there's a big circuit board thing blocking view from the right), you should see the flyback, which is the big black piece of plastic with wires coming out of it that lead to the neckboard:
|The focus pot, viewed from the back of the monitor|
UPDATE (03/20/2015): Someone in the comments asked for shots of it running, so here you go (click thumbnails to embiggen):
As you can see, the scanlines are very crisp and "sterile," though not as much as on a 31 kHz CRT monitor (see the pics at the bottom of this post for comparison). For "240p" non-interlaced content, the PVM sits somewhere between that extreme and a regular SD/CGA TV or monitor, where the gaps between scanlines are almost nonexistent in bright areas.