NB you will need to have Setup>Security Controls>Login Access Policies>Administrators Can Log in as Any User enabled to do this.
1. Head over to the left side panel under Administer', click Manage User / User.
2. This will bring up a page with all users. Under Action click on Login next to your chosen name and account.
3. This will open up a new tab and it will say at the very top of your page what user you are logged in as.