The general rule is: anything perfectly beautiful on print is almost-certainly unreal.
As much as I abhor this rule, and as much as there are untouched images around, it is unfortunate that this conception is the consequence of technology. Ugly people can be made to look perfect, and perfect people are being doubted.
But before you lose hope, here's a spectacular video of how the photographer makes use of one flash to execute 'light painting' over a large subject. No camera tricks or post-processing.