Each state publishes pass rates for NCLEx exam.
What you want to see is that a schools pass rate is close to state and national average over period of years; if below average one year that improvement noted following years.
Here is info from the National Council of State Boards of Nursing:
http://www.ncsbn.org/pdfs/NCLEX_fact_sheet.pdf
Past years summary by state:
http://www.ncsbn.org/pdfs/NCLEX_fact_sheet.pdf
How test works:
http://www.ncsbn.org/testing/psychometrics_faq.asp