iphonemessagessms
When I SMS my friend (iPhone 4 to iPhone 3), green messages mean that it's a Message and blue messages mean that it's an iMessage.
However the messages that my friend sends me (iPhone 3 to iPhone 4) are all grey in color,
How can I tell which message service my friend is using to SMS me?
This is something I wish iOS5 had. Alas, it does not - however if it fails to send as iMessage with send as SMS disabled, it will allow you to tap / hold (I forget which) and specify to send as text.
The main caveat to this, though, is that it doesn't 'alert' you per-se.
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 < Messages < Send As SMS
Best Answer
If the messages are grey then they are being received. The colour doesn't change on the type they use. However if you use iMessage then the message back will be iMessage unless it stats above it that it is a message, as both users need to have iMessage enabled at the same time to use the feature, otherwise an SMS message is sent.
If you scroll up in the message history you will see what the last type of message used was. There will be a line break with iMessage or Message written in it. All messages after that point are dove via this method, until a new break is made.