When you put a breakpoint on and step in, if you look at the call stack it should tell you what module you are in.
So for example the call stack might look like similar to this:
Test.exe!MyNamespace::MyClass::MyFunction() Line 52
Test.exe!WinMain(hInstance=0x01230000, hPrevInstance=0x00000000, lpCmdLine=0x00123456, nCmdShow=1) Line 67 + 0x89
Where the first thing it tells you is the module, we can see here that we are in our own application Test.exe
If your call stack isn't showing you the module names, right-click and then click on
Show Module Names
EDIT: After a quick look myself it seems that the DLL you are looking for is:
Though I strongly urge you to check yourself.