How can I tell where dust is at?

[KmH]

What ever you do - DO NOT SHINE A FLASHLIGHT THROUGH YOUR LENS - because if you do you'll see it has lots of dust inside it.

Lens are not hermetically sealed, not even lenses that have weather seals, and zoom lenses 'breath'. As the lens is zoomed, air moves into and out of the lens. Air that moves into the lens carries dust with it.

 

[Sw1tchFX]

Just to prove that this has NOTHING to do with the lens:

LensRentals.com - Front Element Scratches