Anything that asks you to Update Your Info or you'll Lose Your Account, and then provides some funny looking link where you have to click and log in is clearly fraud.
Anything that states along the lines of "the credit card on file is about to expire, please log in to www.some website.com - with your user ID and password to update it", and then leaves a big long message about never sharing your password is generally safe enough. So long as you're going through the site's main page and not clicking anything special in your e-mail, it's all good.
One giant red flag for scams, which I realize not all online users can recognize, is mistakes in grammar and punctuation. Consider that many of these scams originate in Eastern Europe or Asia. The culprits are apparently not as literate in American English as they would like to think.
Found out the other day that I'm going to be needing a new furnace. I have a propane furnace at the moment, but I want to know which type of heat should I get that will save me the most money on heating bills in the long term ?