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 opt-in 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 for situations in which double opt-in is recommended. 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->Subscribers Settings.
(Note that selecting this option will override any auto-response welcome messages you have set up.)