All of the above also applies to Germany. An additional rule is that you don’t blink before entering the circle, but before exiting so that the driver waiting to enter is informed that they now are able to. This is an important rule and makes a roundabout even more effective. Also, most drivers are disciplined enough to stick to these few and easy rules, in 40 years of driving German roads, it only happened once that a driver took my right of way in a roundabout so I had to do an emergency brake, and he immediately gestured an apology and obviously had some kind of momentary blackout.
I love the YouTubes of somebody launching themselves airborne at speed when taking the straight-through path across the central mound of an unexpected roundabout. Comedy gold. Until somebody gets seriously hurt; that’s never funny.
But how do you tell the ones who know what they’re doing from the drivers who are just confused?
A valid point because we don’t have that many Traffic Islands, and many drivers don’t understand the principles and rules of usage. Of course, when one doesn’t know those things, the sensible and responsible thing is to exercise caution and modest speed. That’s where the idiot part comes into play.. LOL
Look for the crumpled fenders and missing headlights? 
More seriously, for novel driving situations you have a great point. There’s no way to tell the good but unfamiliar driver from the good and familiar driver. It’s often easier to spot the genuinely bad drivers with their crunched cars and erratic decisions.
IIRC, San Antonio has a lot of these, too, and I loved them. I may be misremembering, though. I was only there once, for just a couple of days.
They had these J-Turn intersections in Michigan when I was visiting 20 years ago. When I was told by my friend that I had to turn right to turn left, it was really easy to understand. I was driving around by myself with no problems at all in no time.