How to pick good clementines and mandarins at the supermarket?
Bitterness can come from underripe fruit (although a lot of times they'll just be sour or less flavorful), lack of juice is usually something that has been stored too long and dried out.

A ripe mandarin or clementine should be firm but not hard with a good, bright citrusy fragrance. Rubbing your finger on the skin should impart some citrus oil (unless it's waxed of coated...) I think usually the smell is the best guideline for flavor, although I have still sometimes had a fruit that smells great and tastes pretty bland. But I think usually if they are really fragrant that's a good sign.

The skin should be somewhat loose-feeling. If the skin is tight on the fruit, it is either not ripe or it is drying out (or it's not a mandarin or clementine).

Don't squeeze the fruit and bruise it, but when you pick it up, there should be a bit of give in the skin. Mandarins and clementines have very thin skins, but these still have the sort of spongy texture of other citrus. Because they are thin, they dry out quickly. It's hard to describe what drying citrus skin feels like - progressively rougher and harder than fresh, eventually turning into a crisp and brittle almost stony layer. I'm sure you would notice far before it gets to that stage, but it's not going to happen overnight. (But the fruit shouldn't be squishy or have soft spots, that is generally spoilage or damage.)

It should not feel dry and hard. A juicy citrus will feel heavy for its size, although for really small ones that's kind of hard to judge. 

One more thing to note, though, is that most of the time, the whole batch of whatever type of fruit at the grocery store or at a particular market stall at any given time will very likely come from the same source. While there might be some better and some worse ones, overall they'll probably be pretty similar, unless you can clearly see that some are older or from a different source.

If you're not sure about a batch, just buy a couple and then get more after you see that they're good. (And if you find some great ones, buy a lot and freeze some for juice for later.)