As there is no event to track as a synergy got used there would only be the possibility to hook into the "use" stuff which is shown as the synergy can be used for yourself.
Not sure if this would be possible, maybe not, as noone created some addon for it yet.
There are some addons blocking the usage of synergies, guess by the name of the synergy text.
And others showing you that a synergy can be used:
https://www.esoui.com/downloads/info...Synergies.html
Maybe ask in this addon if they could add another visible indicator for Synergy was used.