What Fiori System (Frontendserver) am I connecting to (ABAP)?
This documents explains how to figure out ( in an ABAP FES landscape) to what ABAP server one is trying to connect.
Presume someone sends you an url:
https://wdciw24.wdf.sap.corp:6021/ui
or
an you need to figure out the ABAP Server / Client you are working in.
Determining system during login dialogue
Solution: On the logon screen, Open Developer tools:
The logon prompt sets cookies, which indicate SAP System name ( UXV ) and (in some cases) default sap-client=395
Determining system after login
Once you are connected, different cookies appear:
The Session cookie indicates System ID and Client.
Note that when navigating further one may open multiple sessions (on different servers)
If there are doubts, which server is the Frontendserver, this information can also be evaluated from the startupConfig.
(in 1.28 this is part of the start_up request response, in higher releases part of the FioriLaunchpad.html:
Set a breakpoint in the FioriLaunchpad source ( in this case the source appears as ui? to the browser). 
Set a breakpoint below the parsing of the oServerSideConfig:
Refresh and analyze the content of oServerSideConfig in the debugger. 
This identifies the Frontendserver system as UXV , client 395.