It's highly improbable that you'll know a number is spoofed until you answer the phone call. If someone is spoofing a number that you're familiar with, you probably won't know that's what's happening until you pick up the call. If the number calling is unfamiliar to you but spoofed using the same area code as your phone number, you can do a reverse lookup on the phone number. If the number is spoofed, the search results will turn up no information. Also, calling a spoofed number back will likely result in a busy signal rather than actually getting to talk to someone.