It could be helpful if you first look if his city can be motivated. If so, what buildings can be motivated? If supplies can be motivated but coin can't, that means you'll be able to plunder his coin for sure but not his supplies at the moment. It's harder to check for supplies and goods as they appear as unable to be motivated even when they're not working (where not working is not the same as disconnected, as you can see that).
This also gives you a few scenarios --- if you can motivate, either all or some of his buildings are currently "producing" and therefore will not be available to plunder at the moment. If you can't motivate anything, his productions are either waiting to be collected/plundered or his city is inactive.
In the first case, you can simply do what Auronus said --- go see if the buildings are "working" (buildings with animations are currently working, and you cannot know when they'll be ready as you have no idea what timing they've used). In the second case, try trial and error --- if he has nothing to plunder, and this happens more than once, consider that player deadwood until proven otherwise.