Rules for stressing Spanish words:
If a word ends in a vowel or the consonants s, n, it is accented on the second syllable from the end: perro, cerveza, yucatecos, secreto.
If a word ends in a consonant other than s or n, it is stressed on the last syllable: pastel, dolor, pared.

There are many exceptions to these rules: many words are stressed on the third syllable from the end, or differently from the above rules. However, the Spanish spelling is very consistent and in the case of such an exception, there is always a graphic accent (dash) over the letter: like this:
haré, salón, gustará is stressed on the last syllable;
lámpara, réplica is stressed on the third syllable from the end.