Inside a subVI, I'm looking for a method to determine if an input is wired from a higher-level VI, or if I'm working with data that was stored in the input control as default data.
You can't really tell from within the subVI is the value you got was from the default value of the control or whether it was fed in by a wire. If this is an issue the best thing to do is to set the terminal to require connection so you know it's been wired. That's what I do with the VIs I write when I want to make sure someone wired in a value. Having a default value basically means that by definition you're allowing the VI to be used without wiring to that terminal. It's the same as specifying an argument in Visual Basic as "Optional".