Go to the Guests page. If you see a blue checkmark next to the message name, the message was sent.
You can also click Reports on the top menu, then click Message Log. Here you will see a log of your text messages. If you see a red highlighted line, there was an error. You should verify the guests' phone number to make sure it's not a landline. Or if the guest is from outside the country, make sure you've added + Country Code before the phone number.