SMS Message doesn't state it has been sent
When the bubble is blue, the message is send as an iMessage. If it turn green, it is send as a regular SMS.

The iMessages have a build in delivery report and is will tell you tings like 'delivered' or 'read' when the message gets delivered/read.  

However, a regular SMS doesn't provide these options, so you'll have to install some 3th party software to get this kind of feedback. So you have to trust your provider to make sure your message gets delivered.
If you have jailbroken your device, there is an SMS Delivery Report repository in Cydia.

If you don't like your iMessages to be sent as a regular text, you can simply turn this feature off in the Settings &lt; Messages &lt; Send As SMS