How can you verify emails from Cox are legit?

I got one telling me my speed was doubled. It had a phone number to call for upgrades. I called it and I got switched to some call center telling me I qualify for a WalMart gift card, yada yada yada. I hung up when they mentioned a shipping charge. It sounded like a scam but the email looks legit so I'm wondering how do you tell for sure a Cox email is really from Cox? Thanks, BGC