How can you tell if an online shopping website is trustworthy?

when on their payment page make sure it is HTTPS and not just HTTP the S stands for secure payment and is there for all trustworthy sites

Do a whois search first, it gives you lots of details about a company's background: http://whois.domaintools.com/ If the results show that a company is located in China and is selling low priced brand name items like for example UGG boots, you know they are trying to scam you. If they pretend to be from a western country but offer EMS delivery, you know that they are located in China too. If they only allow Western Union as payment method, it's a scam. If they hide all important info behind a proxy: scam! If they claim to be a large company but use a free mail account like Yahoo, Hotmail or whatever: scam! Of course you can also check reviews on Google or other search engines, but be aware that many of those reviews are fake and may very well be planted by company employees. If ever in doubt, post your question about a company's background in Yahoo shopping, that section has quite a few experienced users who will help you. Be aware though of fairly new accounts posting a link to another site, quite often those are spambots...

At first I will check the google and alexa rankings of the site it gives the idea weather people are coming to this site, or youc an say weather it is popular or not and does this site is working on order or not google ranking can give a good idea about this and than I will read the reviews about the site in different forums. After that I will observe that do they update their suppliers and gives the suitable rankings to their suppliers and for the confirmation I will contact about 2 or 3 suppliers and ask them about what steps they are taking to secure the money of the customers and if they can satisfy me I will approve the site

read the reviews... google the reviews

I usally look up their headquarter online to see if they're legit!