say I have an app that sends a datamessage with this method
smsManager.sendDataMessage(String destinationAddress, String scAddress, short destinationPort,
byte[] data, PendingIntent sentIntent, PendingIntent deliveryIntent);
say the person who sent it has the app and the person receiving it doesn't have the app, if the person who didn't have the app installs the app is there a way to look in the system for that data message to confirm a phonenumber of the person who originally sent it
...if the person who didn't have the app installs the app is there a way to look in the system for that data message...
In a word, no. Data messages are not handled by the system, beyond receiving. They are merely discarded by the SMS Content Provider, and therefore are not saved anywhere by default. Your app would have to be installed at the time of receipt.