The occluded surface area for each residue is compared to the distribution of occluded surface areas for that type of amino acid in a data set of high resolution structures. The output is found in the file, prot.eval as shown below.
The two important parameters here are Ri and Psr. A plot of Ri tells you which residues are in unusual environments. And Psr should be between 0.60 and 0.74 or your structure is probably wrong.
... Sw: 26.87 THR 316 St: 90.10 THR 316 Ri: 0.30 THR 316 Sw: 52.73 PRO 317 St: 92.00 PRO 317 Ri: 0.57 PRO 317 Total_Res: 317 Pw: 20144.56 Pi: 30304.49 Psr: 0.66
Sw is a weighted surface area of the residue. This parameter is a function of both the occluded surface area of the residue and the distribution of occluded surface areas found in a data set of high resolution structures.
St is the total surface area for that residue type.
Total_Res is the total number of residues
Pw is the weighted surface area of the whole protein
Pi is the ideal weighted surface area of a protein with the same amino acid composition.
Psr is the normalized protein surface ratio. A value between 0.60 - 0.74 is found for properly folded proteins. If your structure has a Psr value outside of this range it is probably wrong.