Quality
First, there is no other way to check it than ordering it. You can do lab testing when you have received the package. Websites that have lab test results can be also faked. They can test good quality steroids but ship you underdosed. So the only way is to order and get it tested for yourself. You will go through sources until you find a good one. The best service and products that supply even top athlete bodybuilders.
Customer support
Not available customer support is another aspect that you should consider. If you can engage, contact customer support and enquire about their delivery terms and so on. If the customer support agents are not only helpful but also respond promptly, you are on the right track. It shows that the business values its customers and is concerned about your welfare.
Avoid suppliers with minimum order requirements
Some suppliers force you to buy more than enough steroids because you are probably not to be satisfied. Such websites have a minimum order requirement. This means that unless you order a certain quantity of the product, they are not willing to sell. Do not buy steroids from any online source with such terms.
Do they care about your steroid cycle and results
Many websites' service is just about selling more products. They will agree with anything you tell them. They don't know about steroid cycles. Find a source that can guide you on your first cycle and give you tips on how to get the maximum out of your cycle. That kind of service is very valuable and it means they care about customers.