I had about a 4 year break from this game, and recently I've came back. There has been a lot going on in ESO during my break and a lot of things are improved.
However, I still haven't seen any improvements when you activate any synergy. Of course this is shown in-game visually (your character's animation) and via sound effect, which is not really a problem in group dungeons. However, in trials, especially now when I've had a break from the game and I'm learning new trial mechanics, it's really hard to tell when and if I actually activate a synergy.
The problem is, there are usually a lot of people standing in stack, so it's really hard to see the character animation. And sound effect, well, disappears under all the other sound effects. And if we take a Mystic Orb as an example, sometimes it may pass you before you activate the synergy, but it's at the same exact moment when you're activating it, thus making you miss it, but I think I did (because I saw the synergy prompt and pressed the synergy key). The only way to make sure I did, is to focus on my resource bars to see does my resource restore anything while activating the synergy.
As a UI/UX designer/programmer I'm familiar with different coding languages, but never seriously used LUA in anything other than few small, personal addons on this game. I tried to look through the wiki to see anything I could use to make a simple text somewhere on the screen to pop up when you successfully activate a synergy, but I couldn't find anything.
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.