The simple answer is that you can't be 100% sure.
Here are 5 browser extensions that automatically expand short URLs for you to check visually if the destination website is familiar. But even familiar sites can contain malware or other attacks like Cross Site Scripting.
Google Chrome and Mozilla Firefox automatically perform checks against the Google Safe Browsing service but this service is not and cannot be up-to-date on all the threats all the time.
Most mainstream antivirus products have some active protection mechanism for web based threats but they are limited. For example, AV doesn't protect against Clickjacking.
Virus Total will scan your URL with many commercial products and services.