What is the easyiest way to spy on someones text messages?

Other than stealing their phone, the easiest way to know what someone else has been texting would be to be the account holder, have access to the account holder's online passWord to their carrier website (ATT, TMobile, Sprint online, etc), log in to the online account and view a transcript of all sent and received text messages, provided they have an online account. Such is not as easy for a prepaid cell phone.