How to know voltage used on a data bus?
If you have a data bus then you'll probably have a clock signal and that clock signal will be about 50% duty cycle. So, using a DC voltmeter you could measure the clock and ascertain the average DC voltage then multiply that by 2 to get the logic 1 voltage (approximately).

If you don't have a clock signal then try measuring a data signal with the voltmeter and usually you will see about 50% of the logic voltage if the data signal is actively sending data.