To identify which syllable is stressed in a word, one must listen to it. A stressed syllable is that which is louder, longer and said in a higher pitch than the other syllables in a word.
In the word “thirsty,” the stress falls in the syllable “thris”
In “guru,” the stress falls in the first syllable too: “gu”
Lastly, in the word “mastery,” the stressed syllable is “mas”
Another way to find out which syllable is stressed is by learning some general rules (although they can not be applied to all words). One of those general rules, for example, is that most two-syllable nouns and adjectives (such as “guru” and “thirsty”) are usually stressed in the first syllable.