Basic Tips:

Copyright Symbol:  If your piece shows the copyright symbol in addition to the Coro mark, it generally means that the piece dates to after 1955.   Many jewelry manufacturers switched from patents to copyrights at about time.  So, it also means that if there is a copyright symbol, it is not as likely that you will find a patent.  Although I believe there is some overlap during the 1954/55 time frame.

Signature / Mark Style:  The Illusion Jewels website has a terrific page with the Coro marks here.  Keep in mind that marks were often used for a number of years.   For example the commonly seen mark of the name Coro in script, was used for many years.  So although it is indentified as 1919, it was used throughout much of the history of the company.