Testing latency is not that hard. You can test your latency with any two servers that are connected via a network.
Perform a series of 'ping-pong' communications between the servers for a set amount of time (eg 10 seconds). Divide the completed 'ping-pongs' between the servers by the time. You now have your transactions per second.
Let's run thru the numbers.
Test duration: 10 seconds
Transactions complete 5,000
Therefore you did 1,000 transaction per second. If your ping-pong transaction was 128 bytes, you did 128,000 bytes per second. That equates to 1,024,000 bits per second. Invert that number (1/1,024,000) and you can say the latency of your transaction was 0.000000978. That's 978 nanoseconds per round trip.
The 978 nanoseconds is your seconds per bit. That's how long it took to send a bit from one machine to the other and back again. That is the definition of latency.
A good tool to use to perform this test is netperf (http://www.netperf.org/netperf/). It will run a ping-pong test over a set period of time and give you the stats I listed above.
On one server, run "netserver"
On the other server, run netperf -t UDP_RR -H -l 10 -- -r 32
You'll get all the info I listed above. You can run the maths with those results.
A shout out to Solarflare's Onload User's Guide. They outline this test methodology in great detail.