I thought I'd put my 2 cents worth in on this.
Electrical systems are all about supply and demand (duh). It is relatively easy to work out what's what by looking at the bikes documentation and doing some calculations.
I found this and am assuming our model is identical to the South African model ->
https://www.motorcyclespecs.co.za/model/bmw/bmw-r1200gs-14.html
This tells me that the bike has a 12V battery (which is odd as they are generally about 13.2V) and the alternator can supply 620W. Ohms Law tells us that by dividing 620W by 12V you can have a current draw of just over 50 Amps. Checking the fuses in the bike it shows that the main fuse is indeed 50 Amps. Sweet.
The problem we do have is measuring what actual current is being drawn at any one instant in time. You can use an ammeter that you place in series and that'll show you. However, what most people do is add up what the various bits are on the bike and go from there. Using Vlad's example, the four driving lights will use 144W on top of the 240W (12V x 20A) at idle which means he's using 384W already. Given you only have 620W to start with, you now only have about 240W left. Out of that you have to send some power into the battery to maintain its charge or it will start providing more current that it can get from the alternator (Fatman and LTP ran into that problem on their Lap of Australia ride). All those other add ons are going to quickly use that spare power so you might get into the process of using some stuff and not others.
Vlads suggestion of using a fuse block is wise. It further separates each add on into its own circuit and that is a good thing.
What I did with my Super Tenere was to install a voltmeter and a Clearwater battery monitor (which I can't find on their site). As your battery usage goes up, the voltage at the battery terminals will drop (on mine from 14.2 down to 13.4 if my spotties and heated vest are on). The battery monitor will show a flashing green colour when there is more current going into the battery than is leaving it (this is good) and will show red if it is the other way around (this is bad, very bad, and you are heading for a flat battery if you let it go on for too long).
One thing that might be useful is a OBD2 reader if the bike in question has that port. My Ford Ranger does and some parameters it does provide is voltage, current and wattage values. I have no idea whether it would work on a bike but you never know.
Good luck.