Is there any test that I can perform to verify that WhatsApp is indeed using end-to-end encryption between my and another Android phone?
There isn't any quick check you can perform in order to be sure that end-to-end encryption is used. Even if you manage to get this confirmation, then you have to make sure that the used encryption keys never left your device (and the device of your friend). If end-to-end encryption is used, but WhatsApp or someone else has access to the encryption keys, the chat is no longer confidential.
There is some available information which can allow a security researcher to start investigating the matter:
- The encryption software is known and the code is open source (even if we do not know what changes were made to the WhatsApp implementation)
WhatsApp will integrate the open-source software Textsecure, created by privacy-focused non-profit Open Whisper Systems, which scrambles messages with a cryptographic key that only the user can access and never leaves his or her device
P.S.: There is at least one way to tell if they are not using end-to-end encryption and parsing the contents of your messages. Some time ago, a security researcher discovered that URLs sent in Skype messages are accessed from Microsoft IP addresses (link). You can try the same thing by setting up a web server and sending some unique URLs on WhatsApp.
I actually did some work on reverse-engineering the WhatsApp protocol a while ago. At that time, it was SSL-encrypted over Wi-Fi and cleartext over 3G. It's possible to root your Android device and install a local packet sniffer to dump the traffic; it's also usually possible to install your own CA root certificate to MITM SSL traffic.
The interesting question will be if it's encrypted within the SSL wrapper, because SSL is device-to-server, not end-to-end.
Sending a message M, should produce a packet P, of data sent, possibly fragmented. If the receiving device does not get P, but instead gets something different, then it's not properly end-to-end encrypted or there is a "tunneling" system going on.
To be really sure, you would have to decompile WhatsApp and check what the code actually does. This may be easier than expected if the Android version is written in Java and is not obfuscated. I have done this before and the decompiled code was surprisingly readable.
Even after that, there may be an (un)intentional weakness built into the system which would enable decryption, such as the infamous Apple's goto fail bug.
WhatsApp messages are already encrypted between your device and their servers, so a packet sniffer won't work. AFAIK they're rolling out the encryption quietly and starting with Android text-only messages first. So the only way I could think of would be to reverse engineer the app. However I think the latest version of the Android app will do end-to-end.