How do you decide what a Main Idea is in an informational text

Make a short list of the details presented in the paragraph and see if you can discover a general theme that ties those details together. For example, if your text said - "they hibernate in the winter, some are brown and others are white, many suffer starvation on account of global warming", the main idea would be - what are key characteristics and circumstances of bears. The main idea would not be - what are the effects of global warming or what animals hibernate.