T/D spread times 400' works just fine for me.
But obviously you have to know both the temperature and the dewpoint for that. The latter typically comes from a METAR or ATIS broadcast, and these typically include cloud information as well.
Alternatively, you can get your own Psychrometer...
Hygrometer - Wikipedia, the free encyclopedia
abgd, it's a (misguided, if you ask me) PPRuNe policy to prevent kids from getting the idea to shine those devices towards aircraft cockpits for fun. This way the thread doesn't show up when you do a Google search.