The following EXAMPLES are randomly made up...
ABCDEFGHIJKLMNOPQRSTUVWXYZ MN are the mirror
BEAST is to GTINM
Look at the 1st letter of word and code, B and G both on the same side [ left of the mirror NM ] therefore NOT mirror codes
SLAVE is to PUNFT
S and P on the same side of the alphabet, NOT mirror code
GHOST is to LDIWM
G and L on the same side, NOT Mirror code
PESTS is to KYOCA
P and K opposite sides, quich check are they mirrors, yes MUST check another pair, look for letters near the end or beginning of alphabet. A from the code is paired with S , so NOT a mirror code.
WEAN is to DVZM
W and D are mirrors, second check A and Z are mirrors, at last we have a mirror, go for it!
Only work out the first and last letters as this will normally be enough to find the answer on a multiple choice answer sheet.
The majority of codes are NOT mirrors, a quick 1/2 second glance will eliminate accordingly.