IOS – How to find out devices associated as iMessage client with the apple ID

The simplest solution is to contact Apple Support (as this article makes clear) since they actually have engineers that can modify which devices and phone numbers are associated with your (or another's Apple ID).

So, just contact Apple Support.

iMessage isn't designed to work with just one Apple ID. Instead of having all the iPhones and iPad on one Apple ID, it will be necessary to set up additional Apple IDs for the different users.

When you setup an iOS device and put in your Apple ID, it provisions ALL such services to that Apple ID. Go into Settings on each device (under Messages and FaceTime) and remove the Apple ID. Replace the Apple ID with a new Apple ID unique to each user. Here is a guide on creating new iCloud accounts for this purpose.

Better yet, provision each account completely with its own Apple ID. If a person wants to download an already-purchased app or song from a different account, you can temporarily change accounts in iTunes or App Store.