The North America Nebula covers an area of more than four times the size of the full moon, but its surface brightness is low so normally it cannot be seen with the unaided eye.
Binoculars and telescopes with large fields of view (approximately 3°) will show it as a foggy patch of light under sufficiently dark skies.
However, using a UHC filter which filters out some unwanted wavelengths of light, it can be seen by the naked eye under dark skies. Its prominent shape and especially its reddish colour (from the hydrogen Hα emission line) only show up in photographs of the area.
Image of the Double Cluster and North American Nebula taken from Kelling Heath by Simon Dawes, with a Canon 600D, Modified with the Full Spectrum Mod and a Clip-in CLSCCD filter