You can do a fair bit of inference from the positive stats. Look how many wars stars they have, how much war loot. Low numbers may (not definitely) mean they tend to not do all their attacks. (whether or not they actually leave)
If they have a timer on them, ask about it. Ask the clan name they left. If they cant give it (like say they cant remember which is generally ♥♥) then take that as ducking the question and don't trust to go into war with them. If they give a clan name ask why they left. if they say lousy in war or don't donate check the clan numbers to see if that holds true.
Also, amount of time on a timer means a lot. Anyone with more than 12 hours is pretty suspect. The closer to going over 24 hours the more suspect they are. Even guys that are fed up with a clans donations or war should wait until the end of one to make their break, and most do. its the hoppers that just up and go on a more often basis.
Note - I have had guys show up with a 24+ hour war timer that it turned out they were not aware of. And some of those guys once it came to their attention doubled back to the previous clan (even though they were mad at them for sucking them into a war they had not wanted) and done the follow through before leaving again and coming to ours. Those are good guys and also proof that accidents do happen.

