A great technique to identify contrast issue as mentioned in this weeks lecture is switching the image to gray-scale and checking the overall range of gray-scale value. Then after identifying areas that need adjustment using the history pallet or undo command, or if the image did not have previous edits before the color change using a “Revert” command would also restore the file to its original state.
I then suggest using the adjustment layers. When I first started using Photoshop we never had these. I would always keep a copy of the original image in a separate layer in case I needed to go back. With adjustment layers this step is not needed. The first tool I always use to adjust contrast is “Levels”. In my experience taking a quick look at the curve will explain most contrast issues. I then simply correct it by adjusting the arrows on the curve.
Another tool I use quite often in adjusting contrast is the curves tool. I also use this tool with an adjustment layer as well. The curves tool has quite a bit more control than the levels adjustment and allows for a more involved correction. One of the main times I go to the curves tool is when I am adjusting an image and removing a colorcast. I great starting point for this adjustment is to click on the mid-tone eyedropper and find a middle gray tone in your image and select it. This may correct the cast, or at least give a great starting point to make further adjustments too.
Resources:
AIO Lecture