To find out if a word is aguda, llana o esdrújula you should listen the word to know which is the stressed syllable and then check the grammar rules: teLEfono ------- the stress is in LE, so it is esdrújula and the grammar says that all the esdrújulas are accentuated: teléfono. CAsa --------- the stress is in CA, so it is llana, and the grammar says that llanas are accentuated when they do not end in vowel, -n or -s....it is not the case....so no accent for "casa". jaMON-------- the stress is in MON, so it is aguda, and the grammar says that agudas have accent when they do end in vowel, -n or -s...it ends with N, so it has accent ------- jamón. Takes time to learn how to accentuated words, even for native speakers. The good news is that a lot of people, in informal situations don't care too much about accentuating words...I guess we are just lazy... http://www.strivendell.com/pages/en/spanish-stress1.asp

http://www.studyspanish.com/accents/rules.htm I hope this clarifies things a bit.