Wifi traffic monitoring at home ?

Hi

We have a Netgear Wifi router at home and I and my wife are both using it, nobody else so private use only.
We are not having out best days in our marriage and I'm suspecting my wife having an other
So what's the easieast way to monitor (spy?) her network usage (in our wifi) in home?
Id like to recall that this is our wifi so that "monitoring/spying" wouldnt be unlegal..

PS: I havent admin pwd to that Netgear and cannot reset the device