how to check the date in the table

Question

Hi all,
am developing desktop application on visual studio 2010 professional using c#,
i have table that have two columns start_date and end_date, these values is taken from DateTimePicker and stored in the table ( like this format 5/28/2012)
now i want to add another item that have start_date and end_date, how can i check is the new item start and end date  is not between any other old item start and end date in the table in the database, 

i mean the start and the end of any item date in the table data is not like any other , every item have unique start and end date
if conflict in date a message is appear that i have conflict in date and i should add a new date start and end that is not exist in the table data
thanks all