Matt,
This isn't the table that I have, but found this on the web.
http://mvddaq.desy.de/~mvddaq/SC/Tdew.html
This has more data points than the table I have (somewhere), but doesn't extend below 0 degrees C for the dew point. I expect this table is calculated from the formulae for condensation rather than reflecting the real conditions which include frost (hoar frost particularly) when the "dewpoint" is below zero...
To use this chart:
Measured temp (dry bulb) is down the left.
Relative humidity (rh) is across the top.
The corresponding dew point temp is in the table.
So if you know the temp and the rh at the same time (have a weather station?) you can find the dew point temp to expect that night. Why? the relative humidity changes with temperature because warm air can absorb more water vapour. The actual proportion of water vapour in the air (absolute humidity) changes very slowly, so we can assume it doesn't change if you check the temp and rh the arvo before observing. So if you know the forecast minimum temperature, you can tell whether it will be dewy or not.
If the forecast minimum temp is less than the dewpoint temp, you will get dew. The lower the forecast minimum is below the dew point, the heavier he dew will be.
If you don't know the forecast minimum temp, you will probably get a pretty good feel just from the rh. High rh, high probability of dew. Low rh, low probability of dew.
Hope this helps. I'll still try to find the other table, though, since it includes dew point temps below zero degrees C i.e. frost.
Al.