tl;dr - look in the Registry.
Packages are registered in HKEY_CLASSES_ROOT\Local Settings\Software\Microsoft\Windows\CurrentVersion\AppModel\Repository\Packages. You'll find the installation folder in there - look through to see if you have .xaml or .js files. HTML apps also have an ~/Applications/<Package> key with a DefaultStartPage value. This will tell you definitively whether it's an HTML app as only these have "start pages".
If the app has neither XAML or JS files, they're likely to be C++/CX apps.
To help, I wrote a small utility which is available on GitHub at https://github.com/mbrit/XamlOrHtml
. This will go through the installed packages and dump out what it thinks it is.
It's most likely to be XAML. My reckoning is that 80% of Windows Store apps are XAML. Interestingly, most of the built-in Microsoft apps are HTML. My view on this is that these are proof-of-concept apps, to prove that you can build Windows Store apps in HTML. At this point, however, if you're building Windows Store apps in HTML... it very much looks like you're doing it wrong.