Or do you travel around from country to country investigating every news story yourself before you believe it?
Do you trust what celebrity physicist Neil DeGrasse Tyson tells you about science or do you read most science papers yourself and spot check by re-running some tests to be sure you get the same data?
We can't all investigate everything ourselves so we have to take someone else's word for it hoping that they've done the research and trusting or not based on what little we know about them.
Now when a politician who can't even understand simple economic laws like Supply and Demand and can't understand how evolution-like processes can produce more efficient economies that are as difficult to muck with as any other naturally evolved ecosystem starts to tell me about science, I don't have a lot of confidence in him. But when someone who almost always makes sense tells me about science I tend to have a little more confidence (and even more if he's an expert in the field).
Either way, I'm basing my confidence largely on the person's reputation (i.e. what I know about him as a celebrity).