Any or all of UL CSA CE TUV ... certification is worthwhile IF GENUINE.
I have seen much Asian manufactured equipment which has had various certification marks attached with no prospect of there having been any related testing.

"Provenance matters" - ie you can usually be confident of certification marks when purchasing certified equipment via a competent & reputable large volume supplier with a good reputation and whose business is founded on professional customer service. Digikey qualifies for that. Sparkfun are trying hard and are good at what they do but I would not class them as qualifying. I'd still happily but product from eg sparkfun, but I'd be discerning about things where safety mattered.[I have no business interests in either supplier - I buy from Digikey occasionally. I don't thin I've purchased from Sparkfun but would happily do so].

If you are buying in any volume then pulling one apart to determine quality of construction is probably worthwhile. A genuinely certified product should pass the "looks OK" test with ease. Anything at all suspect almost certainly indicates a fake certification. Mains voltage clearances should be acceptable, mechanical construction and component mounting reasonable.

A modern regulated power supply should meet voltage spec at rated maximum current without excessive heating. Simply loading one up with a (suitably rated) resistor to max load and letting it "set a while" is a good first check.
Given any two or Power P, Volts V, Current I.
Resistance for maximum load R = V^2/P
P/I^2
V/I

Any or all of UL CSA CE TUV ... certification is worthwhile IF GENUINE.
I have seen much Asian manufactured equipment which has had various certification marks attached with no prospect of there having been any related testing.

"Provenance matters" - ie you can usually be confident of certification marks when purchasing certified equipment via a competent & reputable large volume supplier with a good reputation and whose business is founded on professional customer service. Digikey qualifies for that. Sparkfun are trying hard and are good at what they do but I would not class them as qualifying. I'd still happily buy product from eg sparkfun, but I'd be discerning about things where safety mattered. 
[I have no business interests in either supplier - I buy from Digikey occasionally. I don't think I've purchased from Sparkfun but would happily do so. I think enough of Digikey that if they sell a brand I'd consider it was probably an acceptable one to use, subject to specifications meeting need.]

Note that brands of popular products will often be faked in the general market and that buying through a reputable supplier is usually but not always protection against this.

If you are buying in any volume then pulling one apart to determine quality of construction is probably worthwhile.

• A genuinely certified product should pass the "looks OK" test with ease.

• Anything at all suspect almost certainly indicates a fake certification.

• Mains voltage clearances should be acceptable, mechanical construction and component mounting reasonable.

• Component ratings should be appropriate. Any electrolytics would ideally be 105C (not essential).

• Generally (but not always) a product with fake certification reeks of fakeness internally - but externally MAY look as real as any.

A modern regulated power supply should meet voltage spec at rated maximum current without excessive heating. Simply loading one up with a (suitably rated) resistor to max load and letting it "set a while" is a good first check. Given any two or Power P, Volts V, Current I, then Resistance R for maximum load =

• V^2/P

• P/I^2

• V/I

Suppliers on these sites are more likely than average to offer reputable product:

http://www.findchips.com/
http://www.eciaauthorized.com/

Ones I've not yet tried but which seem OK at a glance.

https://octopart.com/
http://www.oemsecrets.com/
http://www.digipart.com/

REALLY fun places.
YMWV !!!! [You mileage WILL vary]:

http://www.ebay.com/
www.alibaba.com
http://www.indiamart.com/