Another possibility is a mind - reading spell. Harry could peek into Draco's thoughts and memories. However, this is a very invasive form of magic. It could expose Draco's deepest secrets and fears, which would be a huge violation of his privacy and likely lead to a big conflict between them.
He could use a truth - revealing spell like Veritaserum. This would force Draco to tell the truth, and Harry could judge him based on his honest answers. But using such a spell is unethical without proper authorization, which could also cause problems for Harry.
Perhaps Harry could use a divination - related magic. For example, he could use a crystal ball or tea leaves to get insights about Draco's character. This is a more indirect way compared to the previous spells. It might not give as clear - cut answers but could provide some general impressions that Harry could use to form his judgment.