Check a function exist or not in MS SQL Server

SQLfunctionCkeck

IF EXISTS (
    SELECT * FROM sysobjects WHERE id = object_id(N'function_name') 
    AND xtype IN (N'FN', N'IF', N'TF')
)
    DROP FUNCTION function_name
GO
If you want to avoid the sys* tables, you could instead do (from here in example A):

IF object_id(N'function_name', N'FN') IS NOT NULL
    DROP FUNCTION function_name
GO
The main thing to catch is what type of function you are trying to delete (denoted in the top sql by FN, IF and TF):

FN = Scalar Function
IF = Inlined Table Function
TF = Table Function