+1 as above
You can estimate it with the eye...if stars are twinkling above, say, 40 degrees altitude, then the seeing generally isn't worth imaging in, unless your imaging resolution makes it irrelevant. The jetstream prediction is often a predictor of seeing, high velocity jetstream usually isn't good for seeing and vice versa, but there are other local factors that can come into play. Try looking at a reasonably bright star with a grab and go and increase the magnification slowly.
Transparency is how clear the air is...water vapour and particles in the air scatter rays of light resulting in reduced contrast. The same effect can happen when there's a thin veil of high cloud. By eye, calibrate yourself by looking at something like the Coalsack. On a night of poor transparency it can become practically invisible compared with a good night. Transparency is often linked to humidity, but not necessarily. Any nearby light pollution (street lights...) can often be reflected back when the transparency isn't good, making the sky glow unnaturally.
