With your text alert account you can select the option to require a double opt-in for people who subscribe to your messages.
This means that when someone texts in your keyword to subscribe or enters their number on your online sign-up page, they will receive a text asking them to reply YES to confirm:
This option is provided to you to help you ensure that you are compliant with guidelines about opting in to marketing messages. The less you have to worry about messaging guidelines the more you can focus on maximizing your marketing efforts!
You can enable this option under Settings->Subscriber Settings.
(Note that selecting this option will override any auto-response welcome messages you have set up.)