As explained here the default FontFamily used in Silverlight is “Portable User Interface” which resolves to one of the supported local fonts depending on the platform and availability of local fonts.
Is there any way to determine which one is actually used?
Thanks in advance!
No you cant, framework only resolve fonts under hood. But, you can embed/pack fonts in your XAP (silverlight application) with this method you can sure the used font face.
e.g. On Expression Blend 4 http://msdn.microsoft.com/en-us/library/cc294629(v=expression.40).aspx