How do you know if a vowel is short?

Answer 1

If you look up a word in a dictionary, the dictionary will tell you the pronunciation using special symbols placed above the letters - the curved line indicates a short vowel, the straight line indicates a long vowel.

Answer 2

A vowel is short when it is followed by a consonant that stops the sound, resulting in a brief pronunciation. In English, short vowels usually occur in closed syllables, where the vowel is followed by one or more consonants.