Tips for buying vintage cast iron?


I’m looking to grow my collection of vintage cast iron. I was wondering how you search for pieces to add to your collection and what to look for? I know that I should look for the following things to help me confirm that the cast iron is probably vintage:

A made in America label
Heat rings
Maker’s marks

Are there any other tell-tale signs that cast iron could be vintage? It’s been a little overwhelming trying to figure it all out. However, I want to have a short list of things to spot so that I can better sort through things more quickly. Thanks!