I use a couple of different sources, actually. Sometimes I look up Words in my English-Greek dictionary (published in 1910, one of my favorite possessions), sometimes I use words I come across in my translations for class, in which case I use my Liddell & Scott Greek dictionary (or Cassell's for Latin) for the translation, but most often I use the English-Greek (or English-Latin) look-up on Perseus, since its database is the best for searching for portions of words (if I, for instance, want a Greek word for an English word that begins with "aqu")
To check if words that sound familiar have the etymological connections to English that I suspect, I generally employ the Online Etymology Dictionary, as simple as that sounds.