I think that my icloud or iphone has been hacked. I think that someone is seeing my text messages that shouldn't be. How can I tell or how do I fix it
Text messages are not sent to Apples icloud so not possible there.
If you are using Verizon's messaging there may be a way to view your text messages from a computer and another device on your account. But you have to turn on Verizon's integrated messaging and the person must have your password to view them from the web.
Good Luck