You cannot know for sure from WhatsApp alone
Their profile photo may disappear
Their About/info may disappear
Their last seen and online status may stop updating
Messages you send may stay on one check mark for a long time
Calls may not connect
Their chat may no longer show a WhatsApp account name
They may stop appearing in your WhatsApp contacts or chat list
They may no longer respond to messages
The only certain way is to ask them directly