How can I tell if a call claiming to be from Apple is legit?

Nope!

This is the only contact number you should use for Apple.

Contact Apple Support - Apple Support

You are just receiving a phishing message and the scammer hopes you will call and give them your personal information. Apple does not send messages to confirm purchases.

Are you actually trying to make a purchase?

I'd say the answer is probably no.

You can find Apple's US phone numbers here: Contact - How to Contact Us - Apple