How do you know if a place is 'idle'?

*ponders a raid*

You can send them an ingame message. No answer within 2 days, they're idle.

Or watch their population. If there's no growth, they could be making troops, but with smaller populations that's doubtful.

You can also check to see if they have been farmed by anyone else on their village screen under units.

And then there's the scout option. But scouting can be considered an act of war though farming them is the same. LoL!