Experience, really. I guess you mean "they" to mean "native English speakers". If we come across an unfamiliar word, we do follow a few clues to pronouncing it. There are typical rhythmic patterns we follow, plus pronunciation clues from the spelling. For example, if the word looks like it's from (Latinized) Greek, we might assume "ph" is spoken as "f" and "ch" would be a "k" pronunciation: phrenology, geochron. Of course this doesn't work every time, but it works well enough. Sometimes we need to hear the word spoken as well. Unfortunately in English, we've borrowed a lot of pronunciation rules from other languages, plus some words keep a certain spelling even when the pronunciation changes over time. So like I said, experience will indicate the way you probably should say it, if you need to guess.

In the United States, children are TAUGHT to read by a MIXTURE of "sight-reading" and "phonics." FIRST you look at the word as a whole. If that doesn't work, then, SECOND, you try to "sound it out." I don't know how you learn to sight-read. I'm sure it comes with practice. Whole words DO have "shapes" to them. Look at this picture. http://s24.postimg.org/fekizguyd/thelittleboy.jpg Most native English speakers will look at this and INSTANTLY say "Oh, of course--'the little boy!'" (Or "the little lip.") Try taking some cards, writing some very common English words on them, carefully, in lower-case letters, and use them as "flash cards."