You can be confident with secure content through the paypal website for two main reasons.

On the left is the https start to the URL. The 'S' represents Secure. It is part of the nomenclature for Secure Sockets Layer (SSL). Websites that start their page address or URL with http are not secure.
On the right is the lock icon. See below for more details.
SSL uses a complex system of key exchanges between your browser and the server you are communicating with in order to encrypt the data before transmitting it across the web. A web page with an active SSL session is what we mean when we say a web page is secure.
Check that the prefix of the web page address or URL starts with https://, you are on a secure page if it does. If the URL starts with http://, you're not on a secure page.
The other thing to look for is the lock icon. You must see the closed lock icon contained somewhere on the Paypal page in your browser window. (It may be at the bottom of the page).
If you can't find the lock icon or if the icon is unlocked then the page (and the transaction) will not be secure.
Click on the lock icon in the status bar of your browser to view the website details and to view certificates.
Google Chrome lets you know whether your connection is fully encrypted and secure.
Microsoft also has a website information about security.
How to Check If a Website Is Secure? eHow.com