Renaissance Art Query

Hi,

During the Renaissance period the artists often painted some or all of the 12 Apostles.

I remember from school that there was a certain way of identifying them, or that they were always painted in a certain style.

For the life of me I cannot remember what that was.

Do you know, or do you know where I might be able to find out, I have tried exhaustive searches but to no avail.

Any help on this would be GREATLY appreciated.

Many thanks in advance