@dotell
I would say there are some ways, don't know if there is a certain way, a way to sure know who it is. If you think is someone you already know, try to find similarities: This kind of person tries to hide their personality when they go for this kind of thing, but here and there they let something out. Most of the time, they would like that you knew it was them.
Other way would be to ask for social media contacts. A facebook, viber, telegram. If said person is a fake alt account, either they won't give anything or create fake accounts on said medias. Which is way easier to spot.
If this alt account is harassing you, ignore it. It will give himself even more each time he tries to piss you off.