How to find out a finsta belongs to your ex?

I thought this was might be someone I know since they have sent me an anonymous message claiming that the pictures I draw depict him and I. They also continue watching my stories and following and unfollowing my account randomly. I checked who follows this hidden account and several of them also follow my ex whom I had blocked. I have sent them a follow request and they won't accept it. Is it likely him?
How to find out a finsta belongs to your ex?
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