I don't think there is a practical way to proof that the device is totally safe, unless you're able to make it live all the time 'sandboxed' into your own tapping monitoring: your own (portable) BTS to bridge GSM communications and similarly for WLAN.
As, what if the Qualcomm firmware every second full moon and x MB of traffic decides to 'fart out' to somewhere a concise summary of your last communications? It would be a needle in a haystack that not even the sailors who signed NDAs with their providers would know its existance.