If you use the Translink Online Journey Planner, then for journeys involving King George Square station the planner will tell you which part of the platform to use, eg
7.10pm Depart King George Square station, platform 2D
That involves having internet access to look up your journey details, which may not be available.
If you arrive at the station without having the chance to check online, then there is a big bank of screens above the escalators to each platform. These show the next 5 buses to call at each stop along the platform. Depending on the stop and the time of day, this could be anything between 5 minutes into the future and over an hour, so you may see what you need up there, or you may not. All depends on how soon until your bus. The displays look something like this:
Alternately, there is a map of which routes use which stop. Annoyingly, it doesn't seem to be available anywhere around the station entrance, nor just after arriving on the platform. I only found it almost at the far end of the platform, where it's much less use as you've already walked past most of the stop information boards! The photo below is a bit blury, but should at least give you an idea of the information poster to look for if you need it.
So, short answer - if you have internet and/or your bus leaves in a few minutes, you're fine. Otherwise, you'll probably have to walk along the platform checking each stop, unless Translink decide to be helpful and put up some more of the "which stop" posters in useful places...