The au and aw pairs can both make the aw sound as in pause or saw. So how can a student distinguish between the two?

As with so many other vowel pairs, a good rule of thumb is to use aw at the end of a word (or the end of the first part of a compound word, or the end of a word before a suffix is added) and au if it is not at the end of the word.

That general principle, while not always perfect, will work with nine of the ten words found here (dawn is the lone exception).