In the image above, the left coin is an example of the Old Effigy type. The right
one depicts a coin of the Modified Effigy type. The easiest clue is to look at the
initials B.M. on the neck truncation (see here)
Unfortunately, those initials wear often and are not visible. Another way is to look at
the dots of the colons (:) In the Old Effigy, they appear closer together than on the
Modified Effigy coins.
If you look at the colon between BRITT and OMN, you will see on the Old Effigy coins that it is somewhat closer to being halfway between the T and the O whereas on the Modified Effigy coins it is closer to the T.