Most social media sites give official accounts, like companies and high-profile people, a checkmark to put next to their name or username to indicate that they are the only official account, and any others accounts that claim to be the same company or person should be assumed to be fake. However, Twitter is currently undergoing changes to its verification system so this method does not work for this site currently.
If the claims to be a well know source (like CNN or CBS) and only has a few posts in its history, that is a red flag. If it's a well know source and the account has only been active a short time, that is another red flag.
Even though the number of followers can vary greatly according to the popularity of the brand, product or business, it can help you recognize if a channel is official or not. If the account is well known by millions of people (CNN, CBS), the account should at least thousands of followers.
Images of an event are often reused to deceive people by reposting them after the original post was made. You can check if an image has been used before on a reverse image search service like TinEye.