Right now there are only three ways to tell this:

1. Using the detailed logs. I'm not sure if it's available in the client-side log.
2. Using the configuration tool on the grid node server
3. By digging through the grid server registry

So I guess the simple answer is that we don't have an easy way to spot this at the moment. The grid node server will always appear in the list of available nodes, and you can target it with any test, but the tests will just sit in the queue and won't be executed. We have actually been looking at improving this experience somewhat though I can't commit to anything at this stage.