First, make sure you're logged in as an admin. Then, go to the Agents tab, and click on an agent in the list to go to the corresponding agent detail page. Click the Login As User button near the top 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 Log Back In As Admin link at the very top right of every page.
Alternatively, you can log in as an agent by clicking the Agent Activity tab of the Dashboard, then clicking one of the agents' names.