Question

Is there a trick to remembering how to find ionic charges?

Answer 1

Do you mean polyatomic ion charges?
Only brute force memorization works.
Try to remember the most common charges
http://www.iun.edu/~cpanhd/C101webnotes/chemical-nomenclature/images/polyions.jpg

For normal ions: group 1 elements are +1, group 2 elements are +2.
Then I would also remember some common ions, like oxygen (-2)

Check them all out on this page: http://ptable.com/#Orbital
The possible oxidation states are under the element names.