The tarball is just 2.6k, so you won't waste much bandwidth :-)
Jdelay is a JACK client that measures the delay between its output
and input, assuming the channel in between has a linear phase
response (i.e. delay is independent of frequency). If you connect
it to your soundcard and make a loopback from out to in, it will
give you the round-trip latency of your system.
Precision is around 1/1000 of a sample if you have a decent sound
card. Even in adverse conditions (S/N ratio reduced to 0 dB by adding
white noise) it will still measure the delay to within 1/10 of a sample.