This is called "Framework Multi Targeting". There is a SVsFrameworkMultiTargeting service that provides the IVsFrameworkMultiTargeting and IVsFrameworkMultiTargeting2 interfaces. And there is the GetSupportedFrameworks method.
From a package, you can use the following code and then iterate the array of returned strings, where each string contains the Framework + Version + profile:
IVsFrameworkMultiTargeting frameworkMultiTargeting;
Array prgSupportedFrameworks;
frameworkMultiTargeting = base.GetService(typeof(SVsFrameworkMultiTargeting)) as IVsFrameworkMultiTargeting;
frameworkMultiTargeting.GetSupportedFrameworks(out prgSupportedFrameworks);