a) If the email is marked as Spam it is most likely a phishing email.
b) If you have not signed up to a certain service but receive an email about it, it is also most likely a phishing email.
c) If you are unsure, follow these steps (using gmail as an example):
Click the following buttons (arrow then "Show Original"):
This will bring up a page with all of the email's information. At the top of the page, the email headers are present. Check the top addresses for any suspicious addresses that are un-related to whatever your email refers to (i.e blockchain.info). In general you'll want to see the "return-path".
Return-Path: