To determine if the vowel is long when you are reading, look for the silent “e”.  When a word ends in an e, the vowel before it is pronounced long.  For example:

A:  short a…cap   long a with the silent “e” rule…cape

E:  short e…pet   long e with the silent “e” rule…Pete

I:  short i…pin   long i with the silent “e” rule…pine

O:  short o…not   long o with the silent “e” rule…note

U:  short u…cut   long a with the silent “e” rule…cute