How can i be sure?

how do know that my car has a govenor or it just takes a long time to get speed. i red line at 120mph in 3rd gear i put it in overdrive(4th gear) and dont pick up speed. i dont stay on the gas after 120 it takes too long to speed up how do i know that the problem might be a govenor not me being too scared to break 120.(ive been over 120 in other cars mine just takes too long and i realy havent tryed to top it out