Build custom capacitor and measure charge
The easiest way to measure the charge, in coulombs, would be to calculate it by measuring the capacitance of the object (or by calculating the capacitance, given the object's size, proximity to ground and the air's dielectric constant), and measuring the voltage.

Use an electrostatic voltmeter to measure the potential on the sphere, and from that, calculate the charge.

If accuracy is not as important as doing this yourself, make an electroscope and calibrate as a DIY electrostatic voltmeter.

BTW, in air at STP, between rounded surfaces, the breakdown voltage is ~1 to 3 kV/mm. A 1 cm spark to ground indicates roughly 10,000 to 30,000 volts.