Login and open up your own mailbox using a web browser.
Step 1. Click on your own profile icon located on the top right corner of the screen to open the "My accounts" drop down menu.

Step 2. Click on the "Open another mailbox..." link from the drop down menu.Step 3. Enter the desired email that you need to view in the pop up box in the center of the screen. 

Note* Only emails that you have access to will open, otherwise the mailbox will not open.