The spec just states the definitions of auxiliary and primary input sources:

An XR input source is a primary input source if it supports a primary action.
An XR input source is an auxiliary input source if it does not support a primary action

but it does not provide a mechanism for applications to query if an XRInputSource does support a primary action.

Is there such a mechanism, and if not, what is the recommended approach
for applications to distinguish between auxiliary and primary input sources?

Usecase description:

• hand tracking on Quest OS does support select events, so hands are a "primary input source" there.
• hand tracking on Vision OS does not support select events, so hands are an "auxiliary input source" there.
• we can emit wrapped events on Vision OS based on thumb-index distance, but then we risk sending duplicated events on Quest OS (both the wrapped event and then the system event).

Potential workaround:

• treat all sources as auxiliary, these potentially emit wrapper events
• once a source has received a selectstart or squeezestart event, mark it as primary and stop emitting wrapper events.
  While this would kind of work, it still has a risk of sending duplicate events the first time.