If your guinea pig seems listless and hasn't been drinking nearly as much water as usual, check out his nostrils for any visible discharge. His eyes may be excessively watery, or they may appear dull and dry. Any discharge from the nose is cause for concern, especially if you notice a cold draft in the cage area or the bedding has not been changed often enough. Pneumonia is a major killer of guinea pigs, so it would not be a waste of time to consult a veterinarian as soon as you notice any discharge. Digestive problems with a guinea pig are treatable, if not always recognizable.
Most owners notice that guinea pigs can be regular 'poop factories', for lack of a better term.
Guinea pigs are never far from urinating or defecating, so any drastic change in this behavior should be taken seriously. A combination of less frequent urination and lower fluid intake is a strong sign of dehydration, which is another condition especially common in guinea pigs. If you haven't already, purchase a water bottle with marked gradations on the side. Monitor the daily water intake of your pet, and note any major deviations from the average. Again, your guinea pig will put up a brave front even if he is dehydrated, so be objective.