Spying on cell phone text messages is a violation of the Electronic Communications Privacy Act of 1986, so there is no legal way of doing so for free. There are penalties for both intercepting active conversations and viewing stored messages at a later date.
The Electronic Communications Privacy Act of 1986 protects telephone calls and other electronic communications from unauthorized access by both individuals and law enforcement. Actively intercepting a conversation carries harsher penalties than accessing communications after the fact, but both acts are illegal. To legally access an electronic communication, all states require at least one party to the conversation to consent. Some states require all parties involved to consent.
Some states, such as Missouri, carry additional penalties for accessing electronic communications illegally. Commercial software produced to spy on text messages carries disclaimers stating that it is not legal in all jurisdictions and that the customer is responsible for any legal penalties incurred from use.
Implied consent to view text messages and other electronic communications can be given when one party provides the password or other access to the messages. This consent is decided on a case-by-case basis depending on the circumstances surrounding the access. Do not assume implied consent to view a conversation without consulting an attorney.