Underwriters Laboratories provides a service of listing products to verify that they comply with UL safety standards. In addition, the manufacturer is periodically inspected by UL's "follow-up service" to make sure they are continuing to "play by the rules" in the manufacture of the product and the components they are using.
No other third party safety evaluation in North America provides the withering peer review of a UL listing.
Other Nationally Recognized Testing Labs (NRTL's) like ETL (Intertek) also test products to UL standards.
However, for the best verification of a product's safety, especially in high power products like dimmer racks and switchboards, many experts suggest that end users insist on the "UL Listed" Label.
Sometimes, manufacturers attempting to shortcut the UL process will advertise products using terms like:
"Uses UL approved components"
"UL listing pending"
These terms mean nothing in terms of product safety--a product must be listed as a complete assembly in order to pass the rigorous listing requirements of UL.
It is easy to determine if a product is UL listed:
--Look for the label! From http://www.ul.com/marks_labels/mark/mark2.html: "Do not try to reproduce the UL Mark. Only use an authorized UL Mark, which can be downloaded from the UL Web site. You may not use the UL Mark on packaging for products that are not UL certified."
--Refer to the UL 2008 White Book.pdf that itemizes all Listed products by manufacturer and model number.
--Go to Underwriters Laboratories Inc. and select "Online Certifications Directory", where you can search for listed products by manufacturer.