You should mention what OS you're using. Anyway, assuming you're referring to someone using your bandwith via your wireless router, the simplest way to monitor that is to keep a record of all devices that have connected to your N300 router.
I had a quick look at the N300 user manual and found no mention of SNMP support, so you would have to log on to the web interface and check what devices are currently connected, or possibly have the router email you logs on daily basis. You could then easily parse the logs and have a simple list of connected DHCP clients for any given day from which it'd be easy to spot any devices that didn't belong there. Little bit of shell scripting and you could automate it, or you could just collect the logs for a few days and see if there's been any unwanted visitors.
Provided that you've changed the default admin password, any prospective bandwith stealer would probably have hard time avoiding being logged.
Or, in case you don't trust the logs and wanted to take extreme measures, set up a box between the N300 and whatever it is connected to, and set it to monitor all traffic passing through it.
This has nothing to do with the question itself, but just as a sidenote, if you're using a WPA2 key over 10 characters long then no one is going to crack it. They could set up an AP to act as an evil-twin, and you might accidentally connect to that, but then they'd be stealing something other than bandwith. Even if they capture the handsake, the key itself, when strong and varied, would take many years to crack.