Cheapest way to measure .1uL volume of liquid being dispensed from pipette tip
I suspect that the best solution is likely in the physical design of the pipette mechanism rather than in active electronic control based on feedback from the fluid itself.

Another possibility would be to use a stepper motor on your peristaltic pump.

In general though, precise metering of fluids is something that the biotech and medical equipment people have lots of solutions for, and they are usually either based on passive fluid properties (metering pipettes), or closed loop electromechanical drivers (peri pumps, syringe pumps*), but AFAIK not on feedback monitoring of the fluid itself.

For tiny volumes you can do some interesting things with piezo elements as pumps.

Browsing cole-palmer etc catalogs should give some ideas.

Probably you want an entirely separate cross-check scheme for calibration purposes.  If you don't suspect drop-to-drop variation to be an issue, you could just dispense a lot until it should match some easily measured quantity. If you are concerned about variation, a bit more creative thinking may be required.

*or a similar arrangement of a stepper or servo &amp; screw driving lower volume metering device, such as a micro-pipette

If you really, really want to directly measure tiny quantities of fluid, I've seen video microscopy of inkjet nozzles done quite well.  For some fluids you want to backlight, for others you want reflected light at some angle (also consider IR vs visible, a camera is probably fine with either).  Having a known diameter of some feature on your nozzle makes it easy to calibrate the dimensions of your picture.  Once you've achieve a high contrast image, even quite primitive analysis can determine the diameter of a more-or-less circular droplet and from that the volume.