It's a good idea not to check the mixer settings without some prepared test which makes hard for mixer to "fake" proper keying. All mixers have modes where they use only Key Source instead of Key Source/Fill combination. Attached is my own version, feel free to use it anyway you see feet.
If you adjust the mixer settings with only the graphics you use in the event, it may have only elements which are easy to deduce only from Key Source channel, and you can see the problem when it is too late. The card attached has transparent areas in white, black and RGB colors, explicit transparency percentages and large non transparent areas. I usually put it over the colors bars, check on the instruments if color levels match, then put it over live camera to check the transparencies. You can take one glance and be positive your mixer is properly set.
Imgur link http://imgur.com/7LINEcM