Can't do this in vivo very easily as there isn't an Ab with specific epitope for active TGF. You can simply homogenize the tissue and blot for pSmad2/3, but this doesn't quantify active TGF...it simply tells you a relative level of how much active TGF is hitting the TGF receptors on your cells.
In vitro, you can quantify active and total TGF using the PAI-1 luciferase promoter assay developed by Dan Rifkin. This assay is used by many labs to quantify TGF levels in vitro, including mine.