>>509141192
It often pretty easy to find the maintenance pass and take full control. (The factory default? Are you shitting me, vodafone?)
Yeah that isn't any 100% check either, but you'd need to do some mossad pager-tier effort to add in surprises beyond that.
And if you are that paranoid you'd need to start screwing cases open and double check actual components and firmwares on every device in your life.
Besides, consumer WLAN/internet devices are subject to enough hacking and manipulation by bored or nefarious turbonerds that someone would blow a whistle about any really suspicious add-ons which do not belong getting added to consumer shit in large amounts.