If it is to see whether it takes effect I think it is fine if it is to know whether or not to target them I completely agree that it is not fun or fair.
As a DM, I don't think you should be using Power Word Kill at all. "I've set up this awesome encounter that you're all going to love. Except you. You'll have to sit this one out."
There are ways. You could, for example, set up a bbeg where that's his whole deal. The townsfolk are scared of this guy because he has the supernatural power to just kill you, straight-up. Maybe the questline leading up to their encounter involves the players finding defenses or counters or sabotaging his supply of spell components or whatever, such that, if they DO get power-word-killed, it's because they had ample opportunities to not, and failed to take them.
Makes for a great BBEG move to use against a favorite DMPC or other beloved party companion, though. You don't use it on a player, you use it on their feelings.