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Show Wi-Fi status in the menu bar on Mac
Use the Wi-Fi status icon 
in the menu bar to switch between Wi-Fi networks, turn Wi-Fi on or off, or open Network preferences.
On your Mac, choose Apple menu
> System Preferences, then click Network.Click Wi-Fi in the list at the left.
If Wi-Fi isn’t in the list, click the Add button
at the bottom of the list. Click the Interface pop-up menu, choose Wi-Fi, give the Wi-Fi service a name, then click Create.Select “Show Wi-Fi status in menu bar.”
Four signal bars appear in the Wi-Fi status icon in the menu bar. The more black bars you see, the higher the signal quality.
See alsoChange Wi-Fi preferences on Mac