Check the account history of the source. Some red flags to look for are the number of posts and how long the account has been active.
If it claims to be a well-know source (like CNN or CBS) and only has a few posts in its history that is a clue. If it's a well-know source and the account has only been active a short time that is another red flag.
Also look to see if the account is verified by Twitter. Verified accounts are denoted by a blue checkmark badge:
How to evaluate political and current events news you see on social media
What did you see?
What should you look for?