A few weeks ago I was at the grocery store in the produce department making may usual stops and selections. I looked across the aisle and noticed a young woman doing her shopping. She was going down the apple and orange aisle. She had a plastic bag in hand, stopped at the apples and took several apples and without looking at them or anything, just plopped them into the sack. Then she moved on to the oranges and did the same. She simply took the first ones her hand touched. I wondered if she might be in a hurry but she certainly didn't have a frantic air about her. I continued to watch her off and on and that's how she chose all her produce; she just grabbed things and placed them in her bag. It was as if she were on auto pilot.
There is skill involved in selecting produce, especially citrus, so let me get you thinking about a couple of things. When it comes to citrus, weight is a huge consideration. Not all oranges weigh the same. Of course a large orange should feel heavier than a small orange but I'm talking about oranges that are equal in size.
Next time you are at the store in front of the oranges, pick up a few. Even though they are similar in size there will be a considerable variation in their weight. An orange that feels heavier will have a greater water content, which in orange language means more juice. My biggest frustration is an orange that is dry and pithy inside. Ideally it would be great if all our oranges were juicy and sweet but that rarely happens. This weight test will at least solve the juiciness question.
What about sweetness? That's tough. You can't rely on color as an indicator of sweetness because many growers use coloring. In this case you might find that a particular variety will generally give you better results (like a Valencia, or a navel, which are the two most common). Sometimes stores will offer a premium grade, which might be more predictable in it's sweetness. So there will be some trial and error as you experiment.
By the way, the same selection technique applies to grapefruit and lemons and tangerines (all citrus). Because citrus can be a bit pricey I strongly advocate using every inch of the fruit, which means getting some mileage out of the rinds. A microplane is the perfect tool for using every bit of rind.