If you generate the SQL script for a job (tested with enterprise manager), it automatically builds the check for existance and drop statements for you. Example below: -
DECLARE @JobID BINARY(16)
DECLARE @ReturnCode INT
SELECT @ReturnCode = 0
-- Delete the job with the same name (if it exists)
SELECT @JobID = job_id
FROM msdb.dbo.sysjobs
WHERE (name = N'My test job')
IF (@JobID IS NOT NULL)
BEGIN
-- Check if the job is a multi-server job
IF (EXISTS (SELECT *
FROM msdb.dbo.sysjobservers
WHERE (job_id = @JobID) AND (server_id <> 0)))
BEGIN
-- There is, so abort the script
RAISERROR (N'Unable to import job ''My test job'' since there is already a multi-server job with this name.', 16, 1)
END
ELSE
-- Delete the [local] job
EXECUTE msdb.dbo.sp_delete_job @job_name = N'My test job'
SELECT @JobID = NULL
END