In the PlayCanvas editor, I don’t believe there is a gizmo in the perspective view which shows the orientation of the world. (x/y/z) in the manner that Unity editor does.
I’m often wondering if positive X is to the right and Z is into the screen, or is the world X-axis pointing towards me whilst Z is pointing to the left of my screen!
Is this something I have overlooked, or if not, could this useful feature be added? (With the other useful features of Unity’s world orientation gizmo).
That’s a bit hard to understand, but is it simply down to where you camera is located and how it is rotated. This is all relative.
The only difference between Unity’s and PlayCanvas coordinate system is that Unity is left-handed, and PlayCanvas is right-handed. That makes only difference that Z axis is flipped. X and Y are same.
Different engines have different left/right handed coordinate systems, and that is not something you simply change, because all maths in engine is based on it.
3D’s Max for example is right-handed too, but in Max you have Z - up. Which is weirder 
I’m not really asking about co-ordinate systems.
Is there an on-screen gizmo, showing the X/Y/Z axis in world-space? That’s what I want 
Oh, you mean stick a UI Element into Viewport in Editor, that will show world axes based on camera orientation?
Yep, exactly that Max
I am a bit confused by this question.
If you are in the perspective view in the editor and you click on any object while in that view it shows three arrows which point in the three directions of x y an z. Isn’t that what you are looking for?
Just would like a gizmo (UI item) to show which way the world is oriented wrt the camera.
See the gizmo in the top right of this picture of the Unity editor : http://www.raywenderlich.com/wp-content/uploads/2012/11/part1_6b_move_gizmo.png
But you might have local gizmo mode enabled. A ‘view cube’ or reference world space axes would be a constant, onscreen visualization of where your camera is pointing - which is quite handy. All 3D productivity apps do this - except PlayCanvas, currently. 
Nothing best describes than image:
