They know by context. "Metallica is a great band" is obviously a subjective opinion and unprovable. It's the same as "I think Metallica is a great band" but with fewer useless words. This isn't misleading.

If I say something potentially provable that I am unsure of but claim is a fact ("Metallica is the best-selling band of all time"), this would be misleading, and I would be taking advantage of my audience.