The histogram can be seen on the LCD panel on the back of your camera. You have to push a button labeled display (DISP). Sometimes it has a little icon that looks like an LCD screen. Check your manual for sure, as it may also be in your menu. One you bring it up on the screen you want to make sure the data is as close to the left and right of the histogram WITHOUT touching. If it does touch either side but it isn’t a high spike that may be ok depending on the photo.
You can see in this histogram that the data is slightly touching the left side (the shadows) and almost touching the right side (the highlights). This is what you are looking for in a nearly perfect exposed photo. Now, many of your photos won’t have this histogram. Remember, the final decision of how great your photo is remains your decision. The histogram is just one tool you can use, especially when you are out on a shoot, to determine if you should make adjustments and take the shot again. To see a quick video go back to my page and click on VIDEO podcast and choose the Digital Photo I folder and then the histogram video.