For the element, I would pass in the particular object in the web page that you want to verify the font for. So, if it's a particular DIV or button or whatever, the object (Aliases.browser.myPage.myObject.blahblahblah... etc) is the element.
As for the property, in the TestComplete object browser for the object, there should be a property called "currentStyle". If you click on the ellipses beside that property, you'll find an object with a "Fields" tab at the top... go to that tab and whatever property there contains the font you're looking for, that's the property to use.