What exactly do you mean by "check the scalability of an SQL query"? Are you interested in how many concurrent executions you can run, will it cause blocking, etc?

Here's a link to a query stress testing tool written by Adam Mechanic that might help:

http://www.datamanipulation.net/SQLQueryStress/

And perhaps read the following SQLServerCentral.com post on how to post a performance problem and add some additional detail to this thread.

http://www.sqlservercentral.com/articles/SQLServerCentral/66909/