Hi,
Lets say you have the iMessages account and several other such as @mac.com and @me.com and even @icloud.com set up as AIM Screen Names plus a Google one and a Yahoo one.
Then the bottom of the chat where the Text entry field is will tell you which Account you are using.

The List on the left should be the accounts you have Logged in.
The "names" that appear here are the ones set in the Description field in the Preferences > Accounts.
If you are running separate Buddy Lists then they appear in the Window Menu like this.

In my case there are some AIM Registered names as well.
The Jabber.me server has been down for some years now but I have not deleted it yet.
If you don't change the Description field then the account is named as the ID used.

9:02 pm Sunday; December 11, 2016
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 10.x and an iPad (2)