For a few years now, indirect GLX (IGLX) has been disabled by default in xorg and other X Servers. I'm writing an application that will use OpenGL if available, but can fall back to other graphics if it is not. Is there a standard way to detect that it is going to fail, other than trying it and responding to the errors?
My current test (written about 20 years ago) just checks if XOpenDisplay and glXQueryExtension work, but that's not sufficient: things fail later when calling glXCreateContext and other functions.
I'd prefer not to try to open a window and check for success, because at the time I want to do the test I don't know if the user is going to need one. My preference is to do an invisible test at startup so I can warn the user that they're going to be using the backup graphics methods.
Creating a OpenGL context with GLX doesn't require a window. Neiter glxCreateContext nor glxCreateNewContext take a drawable paramter. And even if they did, you can create a window without ever mapping it, i.e. make it visible, or even trigger some action from the window manager.
In X11 creating windows is a rather cheap operation, especially if the initial size of the window is 0×0 and the window is never mapped. You can still perform the whole range of X11 and GLX operations.
The upshot of all of this is, that to test if the OpenGL capabilities are available, the usual approach is to actually attempt to create an window and OpenGL context with the desired attributes and see, if this succeeds.
Since the X11 resources used for probing don't have to be mapped, this will not create any visible output; and apart from constantly polling the X server for the window tree, not even a window manager will take notice (since this depends on mapping the window).
Of course to keep thins cheap and fast, such tests should be programmed directly against X11 / Xlib, without any toolkits inbetween (since GLX is written against Xlib, even if Xcb is used, you'll have to use Xlib, for at least that part, but you'd have to do that anyway).