You can see on a website if the trader is connected with a certain trust mark. In that case there is a mark or symbol on the trader's web site. Check with the Trustmark organization that the trader actually is connected to them.
Note that there are several different labelling systems and that these offer widely varying levels of consumer protection. Check what the current labelling system stands for. In Sweden we have, for example, the symbols "Nordic E-commerce Certification" and "Secure E-Commerce."
If you want to read more about these trust marks, you will find links to the right of this page.