How can I know whether an app supports the iPad Retina display?
Unfortunately there is no way uniformed way to verify iPad Retina Display support. But I wouldn't expect there to be one since Retina and Non Retina iPad applications, from a hardware support standpoint, are identical. Retina Display capable apps will run perfectly fine on the first and second gen iPads. Therefore, there would not be any need for Apple to list this under the requirements section. You're only option is to read the the change log found within the description section of each application. 

I don't think there is any need for you to avoid buying any application simply because it does not support iPad's Retina Display. I would expect any of these apps in question will eventually get the update needed. I would suggesting checking out the last updated date for whichever app you're interested in purchasing. It is always a good sign if the app has been updated recently since it shows active support from the developer. Any app that hasn't been updated in a year is likely one you would want to avoid.

Lastly, a lot of these applications scale wonderfully on the iPad and even if you notice the pixelation from the lacking support of the iPad's Retina Display, that pixelation will be rather minor and hardly ruin the app experience.