These acronyms should help. I use them to teach my Spanish classes.
LONERS --Whenever a word ends in L or O or N or E or R or S, it will be masculine.
Examples: El mandil; El gato; El don; etc
dIONza --Whenever a word ends in D or -ION (all together) or Z or A, it will be feminine.
Examples: La verdad; La acción; La perfección; etc
***This rule works out about 95% of the time
