the model number usually tells the tale. and on some of the older ones the serial number can help. some guys here can nail em perfectly down.. just from a pic!!!
We can tell the year MODEL from model and serial numbers, but outboards, like automobiles, are mostly manufactured in the year BEFORE the model year. Date of actual manufacture is often a mystery, but no matter. . .what we need to maintain them is the model year.
With older outboards it is relatively easy to determine by shape, color, and decals. Newer outboards are not designated by year but by serial number so it is impossible to tell year just by looking.