Voltage by definition is charge difference. Talking about the charge difference of one point is pointless.
That's one common practical definition, and from that, it seems there would be no absolute reference for zero volts. Voltage would always be relative. You'd only be able to measure voltages between two points. All voltmeters would need two leads.
But an electroscope shows that, in fact, there IS such a thing as an absolute reference for zero volts, even though it may be hard to measure precisely. A gold leaf electroscope is a crude sort of voltmeter, with only one terminal.
https://en.wikipedia.org/wiki/ElectroscopeThe two leaves of a gold leaf electroscope will separate if there is a positive voltage on them, and they'll also separate if there is a negative voltage on them. Since they're conductive, they'll each have the same voltage, and since like charges repel, if there is a non-zero net charge on them, they will repel. The two leaves will have no repulsive force between them only if they've got zero volts on them.
Zero volts would be when the body as a whole has no net charge, that is, the same number of protons as electrons.
In practice, an electroscope can't measure small voltages. It requires voltages in the range of thousands of volts to produce a visible indication. It's much easier to precisely measure relative voltages than absolute voltages, and for almost all electronics purposes, the only thing that matters is relative voltage.
But sometimes it's worth remembering that there's a bit more to the story.