Greetings,
Official records, including liens, can be searched at the Clerk of Circuit Court site at http://www.nassauclerk.com/officialrecords/officialrecords.taxdeeds.cfm
Nassau County Treasurer Department Heads and Telephone Numbers including their Delinquent Tax Information Line and the Tax Sale Department are available at http://www.nassaucountyny.gov/agencies/Treasurer/dept_head_phone_nos.html
The Nassau County Department of Assessment records including assessment roll data, district information, tax maps, property photographs, past taxes, tax rates, exemptions with amounts and comparable sales is available at http://www.nassaucountyny.gov/mynassauproperty/main.jsp
The information you need should be available at those linked sites.
Best wishes.
Enrolled Agent
I have prepared individual, trust, partnership, and corporate taxes since 1987.