Bill Fellows gives us an example of polling for a file change using SQL Agent:
A fun question over on StackOverflow asked about using SQL Agent with SSIS to poll for a file’s existence. As the comments indicate, there’s a non-zero startup time associated with SSIS (it must validate the metadata associated to the sources and destinations), but there is a faster, lighter weight alternative. Putting together a host of TSQL ingredients, including undocumented extended stored procedures, the following recipe could be used as a SQL Agent job step.
If you’re going to limit yourself to just SQL Server-based solutions and running on a frequent schedule isn’t enough, this is one of the better options.