UL Numbers From Underwriters Laboratories
"Underwriters Laboratories, an independent firm working with product safety certification, has been active in the field of product testing and preparation of safety standards for more than a century. UL evaluates more than 19 000 types of products, components, materials and systems annually. Every year more than 20 billion UL marks are placed on products from 66 000 different manufacturers. The UL Group and its network of service providers include 68 testing and certification labs worldwide, serving customers in 102 countries."--From the About UL page at ul.com
Essentially, this means that all PSUs sold on the North American market must be marked with a UL number. This number should identify the actual manufacturer of a product. However, not every PSU has such a number. UL number omission on a North American product might indicate poor quality.
Step 1: Reading the UL Number
To find the UL number, there's no need to open your PSU or de-solder anything. Simply open your PC and look at the power supply's label. The UL number usually begins with an "E" followed by a string of numbers. See the three examples below.
If your PSU has such a number, you can move onto the next step, finding out what it means.
Online UL Number Query
Now let's find out more about your PSU. Go to the UL Online Certifications Directory and enter the UL number in the UL File Number field. If the number exists in the database, you should immediately see the result. If not, the number is invalid, fake, or the manufacturer no longer exists.