Hello @Vladimir_Vorobiev1, welcome to the forum!
You can know about how and what index is being used with your query by generating a Query Plan on a query. Use the explain method for that. But, not all collection methods support the explain, and findOne is one that doesn’t allow it. So, as an alternative you can generate the query plan by using the find method for your query.
Useful resources:
- A forum post: Explain function exception with findOne
- MongoDB Manual: db.collection.explain