In order to determine which server your client is connected to, you can query the internal tag "@RM_SERVER_NAME". In the Tag Management this tag is created in the tag group "Redundancy" as soon as you select "Activate Redundancy" in the WinCC project.
Fig. 1: Settings for redundancy
For example, use an I/O field and link this with the "@RM_SERVER_NAME" tag. In WinCC Runtime the name of the active server is then displayed in this I/O field.