When to reveal what you saw using the Map card in Saboteur?
When I have just looked a destination card I often afterwards put a road card that either: 


Obviously leads towards the destination card that I just looked at, or 
Obviously leads away from that card. 


It is then up to my fellow players to decide whether they trust my obvious move or not, based on whether they think I am a digger or a saboteur. I noticed that this is often enough information for the other players to understand what you have seen on the destination card.

I don't think there is a better way that does not render you suspicious. If you give too much information away at a certain point, people will always become sceptical. However if you let them infer the "right" information themselves, they might believe that you didn't intend to give all information. And people consider their own "discoveries" more credible than information that was chucked into their laps. Even if you planned it that way. :)