The service I use in GunSpinningVR to detect controllers availability. `GunController` is the object responsible to controll guns and `UnityVRController` is the object responsible to update position/rotation/button states.

WindowsMixedRealityService.cs

using UnityEngine;
#if UNITY_WSA
using UnityEngine.XR.WSA.Input;
#endif

namespace Demonixis.GunSpinningVR.Controllers
{
    public sealed class WindowsMixedRealityService : MonoBehaviour
    {
#if UNITY_WSA
        private void Start()
        {
            InteractionManager.InteractionSourceDetected += InteractionManager_InteractionSourceDetected;
            InteractionManager.InteractionSourceUpdated += InteractionManager_InteractionSourceUpdated;
        }

        private void InteractionManager_InteractionSourceUpdated(InteractionSourceUpdatedEventArgs obj)
        {
            TryActiveControllers(obj.state);
        }

        private void InteractionManager_InteractionSourceDetected(InteractionSourceDetectedEventArgs obj)
        {
            TryActiveControllers(obj.state);
        }

        private void TryActiveControllers(InteractionSourceState state)
        {
            if (state.source.kind == InteractionSourceKind.Controller)
            {
                InteractionManager.InteractionSourceDetected -= InteractionManager_InteractionSourceDetected;
                InteractionManager.InteractionSourceUpdated -= InteractionManager_InteractionSourceUpdated;

                var manager = GetComponent<GunController>();
                var controller = GetComponent<UnityVRController>();
                manager.SetInputDevice(controller);

                Destroy(this);
            }
        }
#else
        private void Awake()
        {
            Destroy(this);
        }
#endif
    }
}