I have a nice portable "Field-mill gun" for detecting small e-fields on the order of few-cm surface area. Not good for very tiny charged regions. By IDB in UK (Bangor, Wales.)
Look up DIY field mill. It's a non-contact probe. Touching metal to your charged surface will *generate* surface-charges! A field-mill is basically a metal plate with a grounded 2-blade fan whirling in front of it, metal plate connected to a high-Z op amp input. Or just connect the static plate to your scope input and measure the AC peak. As long as the distance to the charged surface is << plate diameter, the peak AC volts is directly proportional to the net surface charge.
A much smaller version would be a metal probe vibrated by a loudspeaker, i.e. the old "wobbulator" capacitor from the early days of FM modulators.
Or, with extreme DC input impedance FET amps, you can eliminate the rotor/vibrator stuff and just hold the metal probe-plate up to your charged object. If the gigohm/picofarad RC decay time through the amp input is on the order of tens of seconds, then just briefly ground the floating probe-plate while it's far from the charged object, then bring it near for a measurement.
Such devices are common on eBay for under $100. Search for "electrostatic locator" or just "electrostatic." The old Simco models are very Ghostbusters-looking. Plus, the input attenuator ahead of the probe plate is a metal *camera iris.*