Look out for signs of positive disengagement. Not apathy, but a generally positive, supportive, encouraging attitude towards everything without any substantial engagement in the content of anything. Also look out for a lot of delegation of decision-making, harmony-seeking and a suddenly increased involvement in useless make-work.

It's basically a version of the lame duck effect. Most people about to leave a job feel bad about pretending to have a stake in activities that will continue long after they've left. This means they don't feel good expressing strong opinions, because people might take them more seriously based on a mistaken assessment of their stake (and implicit promise of future support and contribution to any path they help choose).

While they cannot come out and say they are in lame duck mode, they do the next best thing: keeping things positive without trying to influence anything. Or getting involved in useless things so they can make themselves unavailable for meaningful things.

They do this not just because they feel bad influencing work under false pretenses, but also for the practical reason that they want to keep good relationships going. Idiots who don't disengage and exert a lot of influence just before leaving effectively burn bridges ("why the hell did you push for going with Option A when you knew you were going to leave at the end of the month? You should have let your successor drive that decision.")