A credible source will list the author or authors, and may also include some information on their experience and education. If their experience or education is not included, you should be able to search their name and find their credentials. If an author isn't listed, or if you can't find any information on the author, it might be an indication that the source is less credible.
Expertise comes in many forms. In academia, it means a person has an advanced degree (a Master's or Ph.D.) in their chosen field, and has a history of contributing to that field through activities like teaching, researching, and writing.
Does a chef need an advanced degree to be a professional chef? Nope! They may have a culinary degree, but they will also have a lot of experience working in a kitchen. Does a journalist need an advanced degree in economics to report on economics? Nope! But they probably need a degree in journalism or communications, and some background knowledge on their topic. Does a person who is food insecure need a degree in public health to describe what it's like to be food insecure? Nope! Their lived experience is enough to know what that is like.
The type of information you are looking for might influence what kind of expertise you want to turn to.
Consider what the author's goal is with their writing. What do they want you, the reader, to gain from their writing? The author could be trying to:
Here is an article from a joke news website (clicking the image will take you to the original article):
The article title, "Movie Setting Unrealistic Expectations of How Big Apartment will Be at 30," is anecdotally based in fact. How many movie and TV shows have you seen where the characters are working low-wage jobs while living in large, well-decorated apartments in New York City? That is rarely the reality of living in a high-cost city.
If someone wanted to write more seriously about this topic, they might research the average cost of living in a city and then look at the listing prices for similar sized apartments that appear in these movies and TV shows. In other words, they would provide data to back up their claims.