In the past, many places have advised that the best way to verify if a cashier's check is legitimate is to bring it into a branch of the bank it was issued by. The problem with this is that we are seeing an increase in the number of people being arrested for trying to defraud a bank when they do bring these checks into the branch locations.
So what is a safer option? Verify the check by phone, not by going into the actual bank. This way you can get the information that you need without the chance of looking like you are trying to cash the check and defraud the bank. Make sure that you do NOT call any phone numbers listed on the check itself, because these too could be fraudulent. Look up the phone number online or in the phone book and the call the bank and let them know that you have a cashier's check and would like to verify if the check is legitimate or that you are worried that it could be a counterfeit.
If you are sent a cashier' check for any reason, and are asked to wire any portion of it back to the sender or on to a third person, there is a 99.9% chance that the check is counterfeit.