Basically, you can find each time who a Disguiser/Informant has just become by checking log.
When you do check it, you will find that, on two specific players' logs there is :
"DisguiserInformantName's Role: ???."
This message only appears in general log, and the new disg/info 's log.
Other players' logs will only have "DisguiserInformantName was killed."
So the "trick" is to find which log contains "DisguiserInformantName's Role: ???."
And you've just got your Disg/Info.
It apparently works 100% of the times, so a bit annoying I guess...
I'm gonna attach a replay when I can. (actually any game with a disg or info would work)