Indeed, you never will know it for sure, it is a guess from your feeling. But you can see if a sheat is worked for a keris or not. You are right, sometimes a keris have had more than one dress, one for every day use and one for special use, maybe wedding. And sometimes a expensive hilt was sold because the owner need money and the keris get a more simple hilt. I think that this is also the reason why there so much hilts without a blade.
(good for collecting)
Detlef