You can pick up a boroscope off of ebay or bangood for $10-20, plugs into your computer's usb port or newer smartphone. And as Moh indicated, best for big crud determination but does a pretty decent job of overall cleanliness of the filter. As to how to use - real easy if top shelter is off. If not off, then can pull a shelter pocket off and snake in via that way, or last (and hardest) is snaking in from the front tunnel area (which I honestly don't have a lot of success with). I do a quick inspect every 10-20k or so. By the way, if the top shelter is off, real easy to just remove one additional screw and remove the air box inlet air horn and then shine a flashlight down into the airbox and look with the good ole eyeball.
Double Darksider #847
IBA #54767 BB1500+
Who I was yesterday is but a past memory, who I will be tomorrow is in the future. Who I am now and the type of person I am to others, that is what is important now.