If you want to find out who made a power supply and actually have the power supply already, in many cases you can just look up the ul number. Look for a symbol that looks like a backwards R and then a U (and the two letters are attached). There will be a number under it (typically an E followed by 6 numbers). You can look that up here and it will tell who made most power supplies.
http://database.ul.com/cgi-bin/XYV/t.../gfilenbr.html

Unfortunately, that wont work with antec. If What I have been told is right, the antecs are custom units built for antec by seasonic (older ones are mostly channel well). They are built by seasonic and have seasonic build quality but are models specifically designed for antec. I have also heard that they have lesser quality caps (still very good but not quite as good as a true seasonic). Regardless, antec is a big enough company to have their own registered ul numbers so even though someone else actually makes them, the unit is registered to antec. With may brands that just slap their label on someone elses power supply though, the ul number generally tells the oem.
Fyi I know at least some of the corsair units are seasonic.

Typically I will try searching on the over clocker sites for info on power supplies. If you have some info, you can also usually Google the model and the brand you suspect it to be. For instance I googled "earthwatts 380 seasonic" and this was the third link down (gives a bit about antec and cosair).
http://forums.anandtech.com/messagev...&enterthread=y