Best way to check duplication in database in c#

Question

Hi,

There are many scenario in c# that we need to verify for duplication check before saving records.
So I would like to know that if I have 1 database with 1 huge table filled with around 1-2 lac records say tblcustomer and if I want to check if the customer email ID in it is not duplicate then what is the best way of doing it.
Pls explain with some code if possible.
My .net application would be in c# , i may use ajax if required & backend would be sql server!