If your emulator is running with HAXM, you'll see output like this in your run console in Android Studio, with the relevant line being the "HAX is working" bit:
Waiting for device.
/Users/sbarta/sdk/tools/emulator -avd x86-API10 -netspeed full -netdelay none
HAX is working and emulator runs in fast virt mode
Device connected: emulator-5554

Or, according to Intel's official Installation Instructions for Intel® Hardware Accelerated Execution Manager:
To verify that Intel HAXM is running, open a Command Prompt window with administrator privileges (Run as Administrator) and execute the following command:
sc query intelhaxm
If Intel HAXM is working, the command will show a status message indicating that the state is: "4 RUNNING".