Measure the round trip and divide by two for the approximate one way time. It'd be really neat to measure the time it takes for a packet to travel in one direction, but it's somewhere between hard and impossible[1]; a very short path has less room to be asymetric though.

[1] If the clocks are synchronized, you can measure send time on one end, and receive time on the other. But synchronizing clocks involves estimating the time it takes for signals to pass im each direction, typically assuming each direction takes half the round trip.

You can use something like White Rabbit (https://en.wikipedia.org/wiki/White_Rabbit_Project) to keep clocks in sync. That still involves estimates, but a dedicated time sync network can do things like make sure all the cables are the same length.