I would not immediately assume that your frozen insulin is ok. You should keep a close eye on it, if it appears to develop particles it is bad, but it could go bad and look ok in the pen. With a vial you might also notice it clinging in stringy bits to the sides.
In the end, the true test is whether it loses potency and the only way to really know that is to use it and keep track of things. Unfortunately insulin may not totally go bad, it just may require 2-3 times a normal dose. When I had a frozen insulin incident, my pens looked "ok" because they had not crystalized, but they were not ok.