Testing performance(proactive)

I created a view with cte

WITH UserGroupUserGroups AS (
SELECT
CAST(NULL AS UNIQUEIDENTIFIER) AS ParentUserGroupID,
ug.UserGroupID AS ChildUserGroupID
FROM
UserSecurity.UserGroups AS ug
WHERE
NOT EXISTS(SELECT 'X' FROM UserSecurity.UserGroupUserGroups AS ugug WHERE ugug.ChildUserGroupID = ug.UserGroupID)
UNION ALL SELECT
parentGroups.ChildUserGroupID AS ParentUserGroupID,
childGroups.ChildUserGroupID AS ChildUserGroupID
FROM
UserGroupUserGroups AS parentGroups
INNER JOIN UserSecurity.UserGroupUserGroups AS childGroups ON childGroups.ParentUserGroupID = parentGroups.ChildUserGroupID
)
SELECT * FROM UserGroupUserGroups AS ugugf WHERE ParentUserGroupID IS NOT NULL
As of now i dont have any data into it but i expect millinons of records once this query is implemented. How can i test perofrmance in before so tht i dont get into problems with real data.

Check the indexes available on the table. In the first look you may have Clustered index scan or table scan because you are using NOT Exists.
Madhu