Most, if not all DSLR would have a histogram on the playback screen. If you, like me has been wondering what the histogram means and how to read it, you might want to check out these links.

Readings:
Understanding Histograms
How to Read a Histogram
Understanding Histograms

Videos:
How to Use a Camera's Histogram Part 1 - YouTube
How to Use a Camera's Histogram Part 2 - Wildlife and Nature - YouTube
How to Use a Camera's Histogram Part 3 - Landscapes - YouTube

Credits:
As with my other links, I neither own, contributed nor have the copyrights to these links. Credits to their respective owners.

Mods, not sure if I'm supposed or allowed to post this. If it is deemed inappropriate, please delete this, or inform me so that I can delete it. Thanks!

Others
This lists are not exhaustive and there are many more resources online. So do look up for more materials if you need to, or to add on to this list.

Have fun reading/watching, and have a good weekend ahead.