Since Voltage = work done per coulomb and in a battery we don't know how much coulomb exists (or is it known because I read somewhere that we are unaware) and for the work done I assume we can find it but still we will be missing one of the fundamentals so how is that the voltage is pregiven?
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In practice, we measure it.
A voltmeter is a device which can measure the voltage difference between two places (e.g. the two endpoints of a battery).
The way a voltmeter works makes it so it (usually) doesn't care how much charge is in the battery. The reason for this is that it measures the current that passes through a system with high resistance. Since the current is inversely proportional to resistance ($I= V/R$), the resulting current is very small. This in general does not depend on how much charge the battery has stored, only in the voltage difference between the endpoints of the battery.
