In his table (see below) , all examples seem to start ON the beat, not before it.
Baroque ornamentation usually has a rule that it starts on the upper-auxiliary note, something which is not the case for other eras of music. whether the ornament is on the beat or not is a matter of what fits, what is most musical and what aids in the message of the music.
Except for a few ornaments that are always on the beat you are going to have to use your 'interpret' muscle to decide on how to exactly play the ornaments. That is why some famous baroque pieces never seem to sound the same when you listen to multiple recordings.
Baroque music is richly ornamented but Bach never gave exact meaning to what he wanted from the ornaments, considering the particulars of the playing of ornaments, the performers prerogative.