Once you understand how to read a histogram, it becomes an extremely useful display of the data that your camera is seeing. This 10-minute video from Steve Perry examines what histograms are, how to use them to check exposure, and some tips for avoiding exposure clipping.
Checking for exposure on the LCD screen of your DSLR can be inaccurate depending on the brightness of the screen and the ambient light hitting it. The only truly accurate way to know if your image is correctly exposed is by reading the histogram.
Starting from 1:46, Perry’s video explains how to set your camera up to display the histogram, and then at 2:28 he dives in to an explanation of how to read it.
If you’re shooting in RAW, you have a little bit more room to correct an under- or over-exposed image in post. However, as Perry explains from 6:46, you cannot recovered clipped highlights or shadows. For this reason, it’s important to read your histogram to ensure you’re not losing detail in your image.