It's not that difficult. There's two icons for planets in the outliner, one for a spare building slot, and another for unemployed pops. If both of these are active, that planet can support another specialist building. If only the unemployed icon is active, it needs another district. In some cases you might want another district before more specialists, but that is the basics of it.
There's 3rd one for overcrowding.
But trying to be a good ruler, I tend to plan ahead as I don't want my new pop to sit there homeless and unemployed for 480 days while a building for them is upgrading. To achieve that I have to check all my planets every now and then to see if it's time to order new construction. Also, having some spare jobs and housing increases immigration pull which helps to leech population from other empires if you have migration treaties.
Would be nice if those icon appeared as soon as number of pops reached number of available jobs, not exceeded it. It's trickier for housing as pops can use non-integer amount of it, and you'll rarely be exactly at housing cap - mostly it's just below or slightly above. I guess "show housing icon if the currently growing pop will cause overcrowding" would be ideal.
Guess I'll post it to suggestions forum.
By the way, does anyone vassalize their sectors and if so, what benefits do you get from it? (I love micromanaging planets, so the mere reduction of micromanaging would not be enough incentive for me.)
Actually, you can allow sector AI to build its stuff automatically. But who can trust that silly machine, right?
The thing I don't understand is why we get that "free slot" feeling now. When we had tiles everything that wasn't covered by a blocker was a free slot, but nobody freaked out about that.
It takes time to open a slot now, so getting one is a big deal. We quickly learn to be eager to open slots, so when we get one we feel urge to do something with it.
If you can't help this "gotta fill all slots" urge, but you don't need that many specialist jobs, you might want to plop special resource refineries in those spare slots. You'll consume 10 minerals as income (watch out for that!), but will generate much desired resource and only lose 1 worker per slot.