While the question is rather old and solved, it's still an important and underestimated topic leading to other questions coming up, so i would like to add one approach i did not see in the available answers here, for completeness sake.
Normals (or the orientation of the faces) in Blender can be made visible in 2 ways, one way would be with little lines pointing outward from the middle of a face, specifically the face-normals, while it gives us the information we may need, it can be hard to see wrong directed normals on complex geometry and long stretched faces, if you are a bit zoomed out.
Here i would suggest the Face Orientation to help get an easier overview on where there are faces in the wrong direction. Front and back of the face become blue and red respectively to have a strong contrast and allow better visual identification.
While it is easier to spot with the Face Orientation it does not replace the Normals as those i compared were only the Face-Normals.
But something that happens often is that we work on geometry and apply subdivision surface modifiers to name one that makes most obvious problems with wrong Normals, just to then have to find out that a face got turned for whatever reason.
So i would like to suggest to switch on Face Orientation at least once before giving up on surface problems, most likely if it turns out to be the Normals, the solution is underestimated easy.
Be it with recalculating Shift+N to let Blender figure out what's in- and outside, or by selecting the wrong ones manually and flipping them with Alt+N selecting "Flip". One thing to keep in mind though, both functions need you to select the faces you want to affect!