That is not the correct question. The correct question is “How do I know my cellphone is NOT being tapped?” You always have to make the assumption that your phone, email, etc is being tapped all the time and as a result act accordingly. You can never assume privacy.
When I worked at Cisco I happened to visit a large quasi-governmental company that was buying tens of milions of dollars of storage equipment. I was shown a warehouse sized room filled with storage. It sounded like a jet engine with the amount of ventilation and cooling it required. The workers had to wear headphones to protect their hearing. When I asked what they were doing with all this storage I was proudly told that this company could “re-create any day in the history of the United States since 2003. Every financial transaction; every single email; every text message; every phone call.” If it was sent digitally, they preserved it. Your information is already being stored somewhere. “Tapping” can take place years after the fact. All they have to do is go back into those files and find your data.
The cryptic internet assault group Anonymous has a saying: “Don’t think. If you think, never say. If you say, never write it down. If you write it, don’t sign. If you think it, say it, write it and sign it, don’t be surprised by what happens to you.” Also, playwrite David Mamet said, “Assume that every word you speak on the telephone will be recorded and held against you. Remember, that in practice, the presumption of innocence applies only to the guilty. Should you, God forbid, be tortured, make sure you have something to confess.”
I don’t think truer words have ever been spoken.