The name of the main dictionary used by a buffer is stored in the buffer-local predictive-main-dict variable. Note that the variable usually contains the name of the dictionary (a symbol), not the dictionary itself. This is the dictionary predictive mode will normally search in when looking for completions.
predictive-main-dict can also hold a list of dictionary names. They are then treated as though they form one combined dictionary. However, when predictive-auto-add-to-dict is used, words are always added to the first dictionary in the list. See Automatic Learning.