The player may or may not be given a list of words to search for. Usuaully players are in fact given a list, but this is not absolutely necessary _ in fact the puzzle is much more difficult in the absence of such a list. Puzzle designers who choose not to provide a word list, do usually give at least a clue as to the type or subject of the words in the puzzle.
Once you've found a lot of the letters in the puzzle the best tactic is to look for areas of the grid where there is a lot of blank space. If you don't see any of the missing words, the remaining words in that space are probably reversed or diagonal. Scan for any phrases that match in this area and you're likely to find your remaining words.