How do I know that the microphone is working?

Please note that there won't be any special icon indicating that a microphone has been connected.

Smartphones' headphone jacks were designed for headphones, it's just like the name sounds. So, when you plug in anything other than a headphone, it doesn't indicate that something is plugged in. Again, this is true about all smartphones.

You should be able to tell of the bat if the microphone is working or not because it has a much better quality than built-in mics that come with your smartphone.