Depends on their WhatsApp setting, sometimes if someone deletes you from their contact list, you can't see their profile info like Profile picture, Status message and Last seen if they have set their visibility to "My Contacts" in their setting.
Or use a broadcast list with the contacts you suspect and send a click-bait message. If the message is not delivered to the recipient, then most likely you are no more in their contact list. Repeat few times, to make sure they didn't turned off their phone at that time.
https://faq.whatsapp.com/android/cha...lists/?lang=en
Not a fool-proof, but pretty close.
Or... you can simply call that person all of a sudden and if they ask whose this, then you've your answer.