This post shows how to figure out the voltage of a UART connected to the PS of the Zynq UltraScale+ MPSoC. Figuring out the voltage of a UART connected to the PL would use similar steps.
Look at the schematic to see which MIO UART TX and RX are connected to. Consult this table. Find the corresponding VCC_PSIO pin and what voltage its connected to. That's the voltage of your UART.
1. Examine your schematic and figure out which MIO (Multiplxed I/O) pins the UART is connected to.
For this walk through we'll assume that UART0 RX is connected to MIO74 and UART0 TX is connected to MIO75.
2. Cross check with the TRM or the table below. If the pins are in the this table you're hooked up to the PS UART.
p.599 Zynq UltraScale+ Device TRM UG1085 (v1.7) December 22, 2017
3. Follow the pointer to the data sheet. The data sheet shows that the high voltage is VCCO_PSIO or VCCO_PSIO + 0.30 V depending on the I/O Standard:
The MIO DC input and output levels are listed on p15 of the Zynq UltraScale+ MPSoC Data Sheet: DC and AC Switching Characteristics (DS925) and here:
4. Find the VCCO_PSIO symbol in table 1 to figure out its spec:
VCCO_PSIO is listed in Table 1 on page 2 and here:
5. Go back to the TRM and find the supply pins:
The PS I/O supplies are connected to VCCO_PSIO[0:3] as seen in Table 2-1: Power Pins on p39 of the TRM:
Since UART0 RX is connected to MIO74 and UART0 TX is connected to MIO75 we're in bank 502 and we're connected to the VCCO_PSIO power pin.
6. Find VCCO_PSIO on the schematic and see what its connected to. That's the voltage of the UART.