How to know it's a good deal.

[Josh Dubin]

So please excuse this being such a rudimentary question. But I'm looking to getting into flipping. It's safe to say that one of the most valuable skills a real estate investor can have is not only finding, but recognizing a good deal. How do I know there's a good deal in front of me, without actually getting the house appraised? Surely, I have to be able to quickly sift through dozens of houses to know which are worth looking into more. Am I supposed to look at comps, use Zillow etc? And then, approximately how much cheaper should I be trying to get a house for than what it is estimated to be valued at for the deal to be worth it?