Let's save you have an Android phone.
You write an app for Android that accesses Bluetooth. It should have a value called a 'signal strength indicator', usually denoted as RSSI, that you can access.
You then have your app play a .wav file beep sound according to the RSSI returned.
Or if you have an iPhone, you write an iPhone app. I only have experience programming Android, however . . .
As previous posters mentioned, accuracy won't be that great, but it'll do the job.