When an identity is stolen, how is the crime often discovered?

Your credit or debit card having purchases on your account that you did not authorized. Receiving calls from you financial institute saying their suspicious activity on your account. Getting credit cards you did not apply for. Police arriving at your door for crimes you did not commit. Changes to your credit score. mail not coming when it is suppose to such as bills and other important information. this could mean someone is stealing your mail or had your mail forward to another address. 

i hope this helps