To be totally honest the definitive way is to do a Site Survey that includes a Spectrum analyzer. And even while this is very useful when it comes to intermittent interference (i.e. an engine in a warehouse that only works for 20 minutes a day), it can be very difficult to pinpoint if the issue is related to interference.
As it was pointed before, if you go to Wireless > RF spectrum you can see the actual RF surrounding the AP.
It is also very helpful if you go to Wireless > Access Points and click on the RF tab, where you will see a log that includes channel utilization, clients, usage and most important of all, events, over a period of up to 1 month.