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I've approached the very same topic (see here).

I even started a discussion on the NTP ng about how to best verify the synchronization of clocks. Turns out Chrony appears to be better than NTP.

(Shunning all the infeasible proposals like linking every computer to a PPS source -as if I had any- with equal length cables... way above my paygrade ;-P)

Now what I'm fiddling with is a very small program that does just a ping but on the ROS side of the network rather than via the TCPIP stack (which is bound to be much faster and less affected by ROS traffic, and btw is not directly what we want to monitor). Just that this ping is effectively a message that goes via its own topic. The node has an in and an out topic, this way you can build however long chains by daisy-chaining ins and outs of multiple identical nodes. Only the first one is a master thus doing the timing, the others are just relays.

Now I need to figure out a way to easily assemble data from multiple such chains. I could have each master public a results topic: if all masters used the same topic (using a header to distinguish between them) you'd have to tap only one topic (just like tf_monitor does) but then plotting would be nigh impossible.

On the contrary if you used a different topic for each master, plotting would be easy but aggregating data would be a nightmare...

I'm still in doubt about how to proceed about that.