How can I tell if the man on the phone is really an Apple Security employee?

While on my Mac, I received a 3 screen alert that my personal information had been compromised (or something like that?). It told me to call a specific phone number which was supposedly Apple Security. He gave his name as David **** with an Apple ID # 7*** He told me to call my credit union which I did and spoke to someone, Mike *****, who told me that 3 withdrawals had been made from my account adding up to thousands of dollars. He told me what he wanted me to do then switched me back to this Apple Security man. He told me to leave my laptop on overnight and he would begin fixing things and that I should not tell anyone about this and that he would call me again (many times) and told me to always ask for his Apple ID. This "fixing" process took many hours over 4 days so far. How can I find out of this process is really being conducted by Apple Security???? Help!

[Edited by Moderator]