I think the question raised mori64 really makes sense.
I'having the same requirement: I am working on a "response time" analyzer for a board communicating with RS485 to my labview software (acting as a master), and I need to compute exactly the time it takes from the last message byte sent by Labview software (through an USB-RS485 converter) to the first byte received from the external board reply.
Currently I can only "tick counter" before sending the message to the write buffer, and "computing" the time it takes to transmit all the data (n° of bytes * 10 (8bit+start+stop) / baud rate).
This approach is not precise at all, and it would be nice to know exatly when the last byte has been sent trhough the real comm channel.
However, I think you will never be able to check this from labview since it will depend on the particular hardware you are using to send the data (PCI serial port, usb to RS-something converter...), which is not telling the operating system when its transmission stops.
This situation has been discussed a couple times on the forums (and you may be able to find more details and ideas by searching for some more threads), but the answers usually come back to using a response from your device to gauge the timing. If you're trying to measure the time for that response then it would probably take a call to your device driver to read the buffer status. I don't presonally know the internals of those drivers well enough to know the details of how to make that call, but that would probably be your best option if you can't do it based on a response from the device.