If I've understood, you want to measure the voltage of an RS-232 signal. It's not possible directly. The reception of the signal is made by the UART of your computer and then, what LabVIEW does is to access to its register to obtain the received data, but only at digital level.
If you want to measure these signal levels, you've to use any mean of adquisition signal system
Are you trying to test a UART to see if it meets specs? That would seem to me to be the only reason you need to measure voltage levels. If you're trying to do something else, could explain a little more?
RS232 transmits binary. If you transmit the character A, that gets converted to hex 41 or 01000001 (assuming 8 data bits). There is also start, stop, parity bits. The logic levels for RS232 can between +3 to +12 volts or -3 to -12 volts. When you use LabVIEW to read a serial port, the UART in the pc will convert that digital data back into the character A and that's what LabVIEW or any other communications program like Hyperterminal will read. What exactly do you have hooked up to the serial port of the pc? Is it some kind of instrument or what? If it's an instrument sending data, then how the data is interpreted is dependent on the instrument. It might send back a reading as text. A 1 volt measurment might be transmitted as the ASCII characters "1" and "v". Other instruments might encode the data differently. Measurements made over a serial port can be easily visualized. Plain text received over a serial port is usually displayed as text.
Frankly, that's a completely different problem than what it seemed you were first asking about. While I have a new GPS hand-held, I haven't yet hooked it up to a pc, but, the good news is that there have been numerous threads on that subject. A GPS will transmit data in something called NEMA format. Do a search of the forums (you may have to go back further than the default of 90 day) for NEMA and/or GPS. You should find some helpful information and probably an example VI or two.