Dear Stephanie,
Underexposure and overexposure are subjective terms. Objectively, you can only evaluate how well the dynamic range was used in the shot.
There is no way to determine exactly in Photoshop by how much the image is above dynamic range (that's my way to say "overexposed"), because the information is clipped, and we do not know how much of it is clipped.
For shots that under-employ dynamic range ("underexposed"), look at the white point slider in the green channel of Levels dialogue. White point at 250 is 1/3 eV under, and so on, green channel white point is the left column, exposure adjustment to use the dynamic range of the camera to its full extent is the right column:
250 = 1/3 eV
242 = 2/3 eV
218 = 1 eV
196 = 1 1/3 eV
177 = 1 2/3 eV
159 = 2 eV
143 = 2 1/3 eV
129 = 2 2/3 eV
116 = 3 eV (this is famous 18% grey, 17.78% to be exact)
105 = 3 1/3 eV
94 = 3 2/3 eV
85 = 4 eV
77 = 4 1/3 eV
69 = 4 2/3 eV
62 = 5 eV
56 = 5 1/3 eV
50 = 5 2/3 eV
45 = 6 eV
41 = 6 1/3 eV
37 = 6 2/3 eV
33 = 7 eV
You can use a transparent tape with markings on the camera LCD to take away guesswork.