Also, most spoofs don't use (or link to) the company address or website.
paypal@aol.com is clearly different than
info@paypal.com
There are cases when an email is clearly legitimately from the company, such as when REA sends you an email saying you've been outbid on a lot you bid on.
But, when in doubt, go to the website, login and check there. And just know that there are many spoof emails for many big companies out there.