How can I find out if the number CA have is not fake

Very simple. Just google the name of the debt collection agency. Read the online reviews. Read the Rip Off reports. Check if they have lots of unresolved complaints. If they don't have a proper website or a valid contact details, then it's a red alert.

You can find their identity online. Or if a collection agency is calling you, then ask about their address. If they don't give you their address, then they are fake.

Google is your best choice. Search online by using the name of the debt collection agency and seek out the online reviews. If you don't find a proper website or a valid contact detail, be sure that it is a fake collection agency.