We know because they are audited and certified to known security standards. Google for instance is certified to ISO27001 and SSAE16/ISAE3402 SOC2-II standards. Good enough for banks is good enough for me, and loss or unauthorised access to my data is somewhat astronomically less likely than someone nicking or tripping over my NAS box!
I can tell you have never been present after an audit is announced.
And good enough for banks? Don't you remember those low-tech gangsters from east london who managed to infiltrate a major bank's data with nothing but a USB stick?
Also, you may have forgotten how the NSA/GCHQ initially infiltrated Google? It was accomplished internally by tapping internal data links. That's the reason they encrypt internal data transmissoin these days but it's still not safe with firms moving on to quantum cryptography in light of all the big data corporations using more encryption.