The short answer is, you really don't know. You just have to learn these words as you encounter them. One good example is the pronunciation of "schedule" - generally, in UK English it's a "sh" sound, but US English uses a "sk" sound. Words which come from Greek usually have a "k" sound: chorus, chronological, psychology, chrysanthemum, Christ and so on.

To add to the previous answer, one clue lies in the position of the 'CH' in the word. Words like church, chapel, chafe, chop have a voiced ch. At the end of the word - rich bitch, switch, etc the sound of ch is unvoiced. Say my examples aloud and listen to how the sound changes from the start of a word to the end of the word.