Symptoms of sickness in parrots have to be considered in context. Some are real signs of problems though. A parrot that is not eating. Losing weight. Sleeping all the time. Inability to perch. Vomiting. Discharge from nostrils (nares) or eyes. Excessively watery poops (but keep in mind parrot poops change a lot with changes in diet). Wet feathers around the vent (cloaca ). Any of these by themselves merit a visit to a certified avian vet.
But other symptoms can be explained by other than sickness. Fluffed up - could be happy and just resting. But fluffed up and sleeping all the time - thats cause for concern. Perching on one foot - parrots often do this but if done with also inablilty to perch, thats a cause for concern. Watery poops - if your parrot has been eating a lot of water based foods like lettuce or fruits, the their poops will be watery, but if he eats pellets, nuts and seeds and his poops are watery, thats a concern.
The best thing you can do is to know your parrot really well, his eating habbits, normal poops and his normal weight ( by using a digital kitchen scale), and monitor these a few times a week. That way you will know when something changes.