See Bluetooth Signal Strength

If you use any Bluetooth devices with your Mac, such as a keyboard or mouse, there are a variety of ways you can view the strength of the signal between your Mac and the device(s).

Using System Preferences to View Signal Strength 

To get the most information, you can use System Preferences (Apple menu→ System Preferences). Once it’s open, click the Bluetooth icon, and then select the Bluetooth device in the list while holding down Option
 You’ll see a number along with a familiar signal bar display that you might be used to with your cell phone or Wi-Fi signal.

As with similar displays, five bars is the maximal signal strength, while one bar indicates a weak and potentially unstable connection (the solution is to move the Bluetooth device closer to the Mac or to remove any objects that might be obscuring your Mac or the device, especially those that are metal). To get an updated signal reading, repeatedly click the Bluetooth’s entry in the list while holding down Option.

The number corresponds directly with the bar display and shows the power ratio (dBmW) of the signal. A smaller number indicates a stronger signal—in my test -40 equated to a strong five-bar signal, while -70 equated to a poor signal strength of one bar.

Quickly Viewing Signal Strength on the Desktop

Another way to view signal strength that doesn’t use System Preferences is as follows: hold down Option while clicking the Bluetooth icon at the top right of the screen, before moving the mouse down the menu showing all connected devices (you can release Option after clicking the Bluetooth icon). Here you’ll see not only the power ratio figure mentioned earlier but also battery life (in the case of Apple peripherals) and other technical details, such as the MAC address of the Bluetooth device.

Monitoring Signal Strength Over Time

To monitor signal strength over a period of time, which can be useful when diagnosing connection problems, return to the Bluetooth pane of System Preferences, and select the device you want to monitor in the list. Then hold down Option before clicking the cog icon beneath the list of Bluetooth devices, and select Monitor Connection RSSI from the menu that appears. This will open a new window showing a graph of connection strength, updated every few seconds continuously until you click the Close button at the bottom right.