a) For WhatsApp Business App Users (Manual & Limited):
Manual Checking (Impractical for lists): The only built-in way to check if a number is on WhatsApp is to try and initiate a chat with it. If the number is not on WhatsApp, it will show "Invite to WhatsApp" next to their name in your phone's contact list (after WhatsApp syncs). For a list, this is highly impractical.
Creating Broadcast Lists: If you create a Broadcast List and add numbers, WhatsApp will only deliver the message to those who have WhatsApp and have saved your number. You can then infer which numbers were active by seeing who received/read the message, but this is reactive and not a pre-verification method.
b) For WhatsApp Business API Users (Recommended & Scalable):
This is the legitimate and efficient way to whatsapp number list verify numbers in bulk. When using the WhatsApp Business API (via a Meta-approved Business Solution Provider - BSP, or direct Cloud API integration), you have access to tools that can validate numbers.
How it works: The WhatsApp Business API provides a contacts endpoint. You can send a list of phone numbers to this endpoint, and WhatsApp will return a status for each number, indicating whether it has a valid WhatsApp account.
Implementation: Your development team or your BSP's platform will integrate this feature. You'd typically upload your list (e.g., CSV) to their system, which then uses the API to check the numbers.
Benefits:
Accuracy: Directly queries WhatsApp's system for real-time status.
Efficiency: Can check large lists quickly.