We know that the critical affects for a discustohave a good shape are the genes and the growing conditions from birth.
Following tips are not in the order of importance. Every tip individually has importance in selecting a discus which is in good shape and health.
1) Look for the roundest shape you can while selecting. Any deviation is undesired. For a beginner this is very hard. In my opinion, you have to get experienced by seeing lots of discus at different ages, from different sources. In addition, you can find the official show photos and study the champion fishes of the previous years.
2) Check the nose part. It should be round around the nose and mouth part, NOT two "lines" starting from the upper and lower fins, connecting at the mouth like theblue diamondon the top photo. You can check the desired shapeforthe mouth-nose region on the turq based discus in the bottom photo.
3) Check the eyes. This is very important criteria to get a discus no matter the age is. You can compare the eyes of the two previous fish to see the difference. Small eye/body ratio (just like thesnake based) is wanted to guarantee that the discus grew well from birth. In addition, red eyes are mostly desired. It is a sign of quality in my opinion.
Please keep in mind that the eye/body ratio changes only with the growing situation NOT with the genes. You can still obtain healthy fishes from fish with big eye/body ratio. I say healthy because the fish having a big eye compared to its body is generally stunted.
Stunted discus means that the discus completed its growing period yet is not bigger than let's say 4-5 inches. Stunted discus generally have a weak, oval (not round) body (like a football). On the other side, keep in mind that this situation does NOT effect the fertility. In addition, you can still get healthy babies from a pair having a stunted male/female.
4) All of the fins are important. They should complete the round shape of the fish. They should not start as if they are attached to the body after the fish is born 
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In my opinion thephoto on the right hand side shows how it should be. You can see that upper and lower fin "seems" like a part of the body. In addition, the anal fin has a curvature that fits the round shape. The upper and lower fins doesn't cover the tail or leave a big gap before the tail.
In addition, the tail has to be transparent. Any color on a region close to the body "generally" shows the sign of hormone treatment.
Moreover, a red colored anal fin is an additional quality aspect (which you can see in the photo). Of course remember that genetically some strains might not have a red anal fin.
5) If you see a juvenile discus with almost full coloring and the pattern be careful. Most of the strains start to show them after about when they are4-6 months old and show the full coloring when they reach 12-14 months. Early coloring might be the reason of hormones.
As a result, coloring vs. size is important. Combine this with eye/size ratio information and you can find whether the fish is stunted and/or hormone treated. Of course, you have to observe lots of different strains and fishes togetrequired experiencein order tomake a right decision. I recommend you to spend time in the forums where you can check the discus galleries.
6) Another important point is to check the stress bars. The bars should be separate and must continue from top to the bottom, NOT end at the halfway.
In addition to these body criterias, there are health issues. In the beginning it is hard to identify whether a discus has a serious or temporary illness or a environment based problem. You will get experienced with time as I said previously...
7) The most important thing is to see whether the fish is in good shape or not. If the thickest region is the gill & eye region and if the fish doesn't show the teoritical coloring and pattern, there is possibly a problem with the fish. It probably have the HITH (hole in the head) disease or had this diseaseand moreoverthe fish stunted. To make a decision the particular fish must be observed carefully.
8) The fins might seem like eaten or not smooth. This is probably beacuse of the bad water conditions and not a critical thing to observe, if the fish is great other than this problem. Of course there might be other reasons and you should observe the fish more to avoid any serious problems. My recommendation is to ask experienced people in the forums if something about the fish bothers you. Also try to take a photo if possible.
9) Try to observe whether the particular fish you are after eats different varieties of food or not. Some of the pet shops "like" to feed the fish with worms (from weird sources). As a result, the discus youbuy will possibly have difficulties in getting used to dry food and/or beefheart.
Also, another possible problem will be that the fish will not get the worm as its usual diet(which means high amount of protein). As a result,the parasites passed to your discus from those worms will weaken the fish, while your discus is trying to get used toyour diet system. Then, in a couple of weeks, you will have to fight with diseases instead of growing the fish. Thismight result as a stunted discus IF the fish you bought isa juvenile fish.
10) Check whether the discus is using both gills or not. If the fish holds & not usesone of the gills, this means that the fish has gill fluke disease. Gill fluke is a serious disease if left untreated.
11) A healthy discus swims with all the fins erected as you can see in the photos at the top of the page. Otherwise means a problem most of the time.
12) A shy discus doesn't mean a sick one all the time. Sometimes the environmental issues makes the discus shy temporarily. Lighting may be a reason. Shadowscreated because of wrong lighting increases the stress for a discus. Any vibration in contact with the tank will create stress. Sometimes an unwanted tank mate might disturb the discus and this will make the discus a bit shy. These are not serious problems as you can guess so please keep every possiblity...
