Signals that your dog isn't feeling well can range from the subtle (things like sleeping more than usual or being lethargic) to the super obvious (vomiting, diarrhea, unconsciousness).
You are the ultimate judge of whether your dog is feeling under the weather. You know your dog better than anyone else, so if your guy is acting in a way that is not normal for him, he may be feeling sick.
Most of the time you can simply monitor your dog until the vet opens on Monday morning, however if your dog should develop seizures, bloody diarrhea and/or vomiting, blue-tinged lips/tongue or listlessness, then you should seek emergency treatment right away.
I hope this helps.