To know if someone is on a WhatsApp call, you can try the following methods:

1. Look at their WhatsApp profile: If someone is on a WhatsApp call, you may see a green phone icon with a timer next to their profile name [1].

2. Call them: You can try calling the person on WhatsApp. If they are already on another call, you will hear a message saying that they are on another call [1].

3. Check their online status: If the person has been online for a while and is not responding to your messages, they might be on a call [2].

4. Check their last seen status: If the person's last seen status is from a while ago and they haven't read your message yet, they might be busy on a voice or video call [2].

Please note that these methods are not foolproof and may not always accurately determine if someone is on a WhatsApp call. It's always best to directly communicate with the person to confirm their availability.

Learn more:

1. [Best 7 Ways] How to Know if Someone is on WhatsApp Call?
2. Answered! How to Know If Someone is on WhatsApp Call
3. [Without Calling] How to Know Someone is on WhatsApp Call?