Discus fish are pretty sensitive. They can become unhappy pretty easily since they are always demand high standards. Also they have ways of showing that they are not Happy. This is why its hard to keep Discus fish and why they need lot more attention than any other fish. How do you know if you are Discus fish are happy ? There are plenty of ways to know this.
Discus fish will show bright colors - They will be having brighter colors but not darker colors. If they look usually dark it means they are in stress. This can happen after a water change or when you move fish to another tank. Also if tank is low on oxygen this can happen. Also you can see them in dark colors and stressed when they are harassed by other Discus fish or other fish. So basically when they have bright colors and showing off their Beauty like a peacock that means they are happy and If they are become dark and don't swim like usually that means something wrong.
If Discus fish are not Shy - If your Discus fish are coming near to glass and ask for food that means they are happy with everything. Sometimes they can be shy first few months after you put them into your tank. They start coming near to glass when they get used to the tank. I know some say Discus are skittish fish but not when u stay near them everyday. they get used to you and start come front of glass when they see you near.
If Discus fish are active - If you see your Discus fish very active and even fight with each other it means they are fine. They might not swim a lot but if they are active they won't stay at far end of the tank. You can see them fighting specially at feeding times. They fight with each other and rush at food means your Discus fish are happy.
If you Discus fish have a good appetite - It's a popular myth that Discus fish don't eat a lot. They have a big appetite and young Discus can be fed upto 6-8 times a day. And Adult Discus fish can be fed 2-4 times a day. If your Discus fish are rush at food and eat then they are happy. If they have no interest in food or don't come near glass even when you feed then you should worry about your Discus food. They might have a illness like white poo problem.
Discus fish need lot more attention that your other fish need. You have to watch them and see if somethings wrong. To keep Discus fish happy you have to careful about food you give them, Water quality, oxigian level of the tank, temperature.
Discus Fish - Complete Pet Owner's Manual is one of the least expensive book available on the subject of Discus fish ! But it packs lots of useful information about Discus fish keeping. This is great reading material for those who want to have Discus fish and who already have Discus fish not lack of experience. It covers all the basic areas you should know but doesn't have too many info which might bored you.