Summarize this article:

If your dog is trying to vomit, and cannot, this can be associated with the life-threatening condition, bloat.  Get your dog to the vet immediately in order to ensure the best outcome. If your dog is trying to urinate, but is unable to do so, take it to the veterinarian quickly.  Inability to urinate indicates some form of blockage and can be quite serious. If your dog is having trouble standing, is swaying, or has collapsed then you must seek care quickly.  Even if your dog is tired, it will be able to stand and move.  Collapsing is a definite sign that your dog needs medical attention.

Summary:
Watch for non-productive retching. Monitor for non-productive urination. Look for any weakness.