Is it possible to ask Siri to read dictated text?
Are you just using the dictation feature (the microphone button on the keyboard), or sending email or texts/iMessages through Siri (holding down the home button, telling Siri to send a message…)?

If you're using the Siri function, when it prompts you if you want to send it, you can ask Siri to "read it to me" or "review it" and it will be read back to you.

If you're just using the dictation (which technically isn't Siri), your best option is to use VoiceOver, as Andrew Ferrier suggests.