When using a repeater the repeater usually will have the same SSID as your WiFi AP. However, it will always have a different BSSID (think of it as of the MAC address of a network interface).
So, in order to tell to which device you're connected, you have to find a way to display the BSSID you're (currently) connected to. While I was able to find a command for windows which will show you all the known BSSIDs of a SSID:
netsh wlan show network mode=BSSID
I was not able to display the BSSID which is currently used (eventually someone here is aware of such a built-in command).
However, I found a tool which will show you the available BSSIDs and make it possible for you to connect to a specific BSSID: checkout NetSetMan at www.netsetman.com .
Once installed and started, go to "Tools"-->"NSM WiFi Management". You'll see all SSIDs listed and if a SSID has multiple BSSIDs - as it will if it's using a repeater - you can look at those BSSIDs if you hover over the SSID with your mouse.
If you're on Android you can use Apps like "WiFi Analyzer" which will also present you with the BSSID.