Actually, I don't think it's possible.
Assuming the air has the same moisture content all the way up to the cloudbase, which I would think is quite unlikely, it might be possible if you knew the lapse rate. ISA gives us a lapse rate of 2 degree per thousand feet, but we know that ISA usually doesn't match the real world. The adiabatic lapse rates we learn for our exams don't help, since we're not talking about adiabatic lapses here. The type of clouds may give you a bit of a clue: cumulous clouds indicate unstable air, in other words a high lapse rate, so you'd expect a lower cloudbase than you would for stratus cloud and the associated stable air, if the temperature and dew-point were exactly the same, but I don't think that's going to be very helpful.
If there's any kind of frontal activity, then all bets are off, because there will be a different air mass at altitude to that at ground level, with different associated temperatures, lapse rates and moisture contents.
The best way of finding the cloud base is either to look at the TAF/METAR, or to fly up to the base of the clouds (remembering, of course, that you need to have 1000' vertical seperation from clouds above 3000' or fly IFR).
FFF
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