A static method used to check if an HMD is available with the OSVR SDK for Unity.

CheckOSVR_Unity.cs

public static bool HasOSVRHMDEnabled()
{
#if UNITY_STANDALONE
    var clientContext = new OSVR.ClientKit.ClientContext(GamePrefs.AppID);
    
    // Check if the server is running
    if (clientContext != null && clientContext.CheckStatus())
    {
        // Check if the HMD si connected
        var displayConfig = clientContext.GetDisplayConfig();
        if (displayConfig != null && displayConfig.CheckDisplayStartup())
        {
            DeviceType = VRDeviceType.OSVR;
            displayConfig.Dispose();
        }

        clientContext.Dispose();
    }
#endif

    return DeviceType == VRDeviceType.OSVR;
}

CheckDisplayStartup may return false initially. You may need to update the clientContext a few times. I typically something like while(!displayCOnfig.CheckDisplayStartup() && !TimedOut()) { clientContext.Update(); Thread.Sleep(100);} or something similar.

Also note: It's our intent to abstract HMD vs Monitor vs cave/etc... in the DisplayConfig API to enable the widest possible supported environments with roughly the same code. You may encounter one of the following configurations (not an exhaustive list):

A PC monitor: one viewer, one eye, one surface, surface size = displayInput size, no distortion.
A 3D monitor: one viewer, two eyes, two surfaces, both surfaces = displayInput size, no distortion.
Cave: one viewer per wall, two eyes per viewer, surfaces = displayInput size, no distortion.
HMD: one viewer, two eyes, one surface per eye, surface size = half of displayInput size (possibly upscaled for distortion correction), distortion according to the HMD config.

The last scenario is roughly equivalent to "Is there an HMD attached?", but I would caution filtering out the other scenarios unless absolutely necessary. The real question you're interested in is not that there is an HMD attached, but is there is a head-tracked viewer. That tells you whether to adjust the UI and controls for an HMD-like experience (i.e. draw the UI in world space, disable mouselook, etc...) vs a 2D monitor experience. Unfortunately OSVR doesn't yet have that information explicitly. If you don't care about 3D monitors though (which have two eyes but aren't head-tracked), then you can just check for the existence of a viewer with two eyes and treat it like an HMD.

Hum thanks a lot for these explications!

So this code must be better right?

var isHMD = displayConfig.GetNumViewers() == 1 &&
                    displayConfig.GetNumEyesForViewer(0) == 2 &&
                    displayConfig.GetNumSurfacesForViewerEye(0, 0) == 1;

I would leave out the GetNumSurfacesForViewerEye check unless you're implementing a rendering plugin or engine backend and can only support one surface.

Also, a correction: A cave system would have one viewer, two eyes, and multiple surfaces per eye. You'll probably be ok treating a cave system like an HMD for UI/control purposes. You definitely don't want mouselook on in a cave! :D