So, suppose there is time dilation between Karag Dum and the rest of the world. Karag Dum, as a waystone, collects magical energy at some rate in its own frame of reference and sends it south to Karak Vlag by relay waystones not adding anything on their own. One tendency of a magic is that it is loosely follows law of conservation of itself (else wizardry would look differently, I guess). Therefore, amount of magical energy that Karag Dum sends out in X years since its isolation loosely equals amount that Karak Vlag gets in Y (=185 now) years that passed in outside world, therefore actual observed flow of magical energy is X/Y times the nominal flow that Karag Dum should be outputting. General chaosiness of landscape around Karag Dum might influence the flow, but it would be just usual "magic has tendencies, not laws" error margin but somewhat bigger. Therefore, by comparing waystone flow to Karak Vlag and some other waystone flows within Dwarven network used as reference Mathilde could discern whether Karag Dum is subject to severe time dilation, on level of order of magnitude, or not, assuming of course she ever observed any other waystone transmission lines anywhere.
...too bad I got this idea only when we are already at the gates of Karag Dum in-quest and irl by update waiting time.