Well they thrive in low humidity and on the undersides of leaves. And like I said, in minor outbreaks you might never notice that they are there.
If you take a suspected affected leaf, turn it upside down and look on a sharp angle, down the length of the leaf rotating the leaf slightly, you might see a "silvery sheen". That indicates their presence.
From above leaves like Oncidiums, Paphs., Miltonias or Catasetinae will look sickly on first glance. They have an odd pick marked appearance.
