You can use threshold to find the lightest part of your image (you want the lightest part with detail, not a reflection or specular highlight) and darkest part.
This comes in handy when you use the eyedroppers in your curves dialogue box.
To do a quick correction for contrast you can use the black eyedropper to set your black (anything the same tone as the pixel you click on or darker goes to black) and use the white eyedropper to set your whites.
Before we get back to threshold heres what you should do.
Go into layer - new adjustment layer - curves. Once you are in the curves box, double click on the white eyedropper. This will bring up the options for this tool. By default it is set to pure white (255) and we want to set it to give us white with detail.
Here are the settings you should start with:
H:0, S:0, B:94, R:240, G:240, B:240, L:95, a:0, b:0, C:4, M:3, Y:3, K:0.
If you think that after using these settings your whites are not white enough just change the R, G, B, numbers higher (up to 249).
The same goes for the black eyedropper (default is 0), if you want some detail just move the R,G,B, values up between 10-20.
So now that all our options are set and we know how the eyedroppers will work we can go back to using threshold to find our lightest and darkest area.
Go to image - adjustments - threshold.
Move slider all the way to the left. Now bring it back to the right until a bit of black appears. This is the darkest part of your image. Shift - click on that part of the image to set a color sample so you will know exactly where the darkest part is.
Now move the slider all the way to the right and then bring it back until some white appears.
Shift - click in one of these areas to set another color sample.
You can now cancel out of threshold and you should see to targets on your image.
Now go to your curves dialogue box and shift - command - click on these target areas and it will set these points on your R, G , and B curves.
You can now adjust these areas.
To clear the color sample targets just go the the color sample tool (in toolbox under the eyedropper) and go up to the options bar and hit clear.
These keyboard commands were for a mac computer.
Sorry to be so long winded tonight.