Open WhatsApp
Go to the chat with the person
Check the top of the chat for their last seen status
If visible, it will show the time they were last active
If it says online, they are currently using WhatsApp
If last seen is hidden, you may not be able to see it
Check your privacy settings and theirs
Make sure you have not been blocked
Make sure the person has not disabled last seen visibility
Update WhatsApp to the latest version