How do you determine actual latency?

I’m currently using Quartz, an addon to replace the default action bars. This addon has a feature that shows you how much time of the cast is delayed due to lag. I notice that it always does not match with the indicated lag when I hover over the game menu icon.

I notice that the tooltip of the game menu shows that my latency is 330 ms 90% of the time. However, quartz shows that it’s higher most of the times. It’s within the range of 250-500 ms (variance is so real as its quite dynamic), and I’d say that it’s around 400 ms on average.

A while ago when i did a key, I noticed that the delay was significantly longer but the tooltip of the game menu still says ~330 ms which I think is quite inaccurate. It personally felt as if the delay was a half a GCD so around 600ms - 700 ms. Quartz seems to be more accurate here in indicating the lag.

So how do you determine (in time unit) the true lag you’re experiencing? Are traceroutes or pathpings more reliable?