Each bluetooth device has a unique 48 bit address and a device name (up to 248 bytes). The name is usually shown when doing a bluetooth scan with a mobile device. The only way you can tell the bluetooth device is an OBD scanner is by checking it's name or address, the fact that it's an OBD scanner does not give it any special properties from the point of view of a mobile device.
To put it simply, you do a bluetooth scan and check the found device names. The name usually reveals that it's an OBD scanner, but it may differ depending on the manufacturer. If you want to use it with one specific OBD scanner that you have - just find and store it's name or address.