The most straightforward and easiest way to test the soundstage of your headphones is to listen to songs with incredible vocals and instrumental performances. There are many songs that can help you identify whether your headphones help you create a good soundstage or not. I would recommend listening to the song Bubbles by Yosi Horikawa. It is a great song that can help you test the soundstage in your headphones. In fact, in my opinion, anything by Yosi Horikawa creates a great soundstage.