I sometimes check their wall. If they're inactive, then they usually won't come up in community lists and their neighbours will have stopped visiting. Their most recent comment will be months or a year ago.

Personally, I fully expanded so I've deleted virtually all my neighbours except for the ones I know for certain I got map requests from recently and those gifting gold to me. It is massively slow deleting neighbours btw. I think TL don't like social connections being cut so the processing takes a several seconds. it's worth it though since my neighbour list now loads super fast