to reveal it as part of the plot contained within the dialogue would be pretty difficult
there is a little remembered scene in epsisode 2 when yoda, windu and ki adi mundi are in a meeting with palpatine in his red office when padme enters with her entourage...palpatine says something to her and yoda turns around and gives him a suspicious look...
i think yoda knew then about palpatine and the darkside were bedfellows
i guess they just didnt reveal it because it would be better timed to coincide with anakin turning to the dark side
if it was revealed earlier within the plot then it would most likely have meant that sidious would have been killed as anakin would not have been around