* is there an easy way to see what toolkit an application is using, just to be sure we aren't looking at applications that are using older versions of QT or some other toolkit?

"About" -> "Qt" menu should work for most applications, I think.
Here, QJackCtl, for example, says: "This program uses Qt version 4.8.6". And if I remember correctly, a bunch of others Qt applications have the same menu.