An agent will normally be scheduled to run automatically. At any time however, you can manually run any of the queries associated with an agent by clicking on the name of the agent on the agent hq screen, or on the text that informs you of new items found.
If you click on the name of the agent, all items (including new items) will be included in the query. If you click on the 'new items' text, this will include only the new items in the query.
For more information on resetting agents, see the New results - checking your agents section.
Note: - when running the search in this way, only those items that the agent has found since it was last reset will be displayed.