Some questions to ask yourself:
1. For journal articles: Is the journal it's published in peer-reviewed?
2. For journal articles: Can you tell the reputation of the journal, such as *Impact Factor or other indication of the journal's standing?
*“The journal Impact Factor is the average number of times articles from the journal published in the past two years have been cited in the JCR year. The Impact Factor is calculated by dividing the number of citations in the JCR year by the total number of articles published in the two previous years. An Impact Factor of 1.0 means that, on average, the articles published one or two year ago have been cited one time. An Impact Factor of 2.5 means that, on average, the articles published one or two years ago have been cited two and a half times. Citing articles may be from the same journal; most citing articles are from different journals.”
From https://osu.libguides.com/c.php?g=110226&p=714742
3. Who wrote or otherwise created the source? What can you find out about that person?
4. How many times has the source been cited?
What does this tell me?
5. How current is the source? Does it matter for your topic area?
"Popular" sources are things like magazine and newspaper articles, blog posts, or other types of media such as podcasts. They are often not created by an "expert" in the field they're covering, and you often won't know where they got their information.
Healthy skepticism is a good thing.
If you can't answer these questions, investigate!
You must determine what is appropriate for your topic area or assignment.