First, make sure you're logged in as an admin. Then, go to the Users tab, and click on an agent in the list and click the Login As User button on the right to log in as that agent.
You will be taken to your chosen agent's Dashboard, and your view of the entire lead manager is the same as that agent's. This means you will temporarily give up your admin privileges until you log back in as admin. This can be done at any time by clicking the Return to Admin link at the very top of every page.