You are right, I don't think they know. I seen one investigator at a tornado site trying to determine the strength of a tornado.. somewhere. He came to the instantious conclusion that is was an F5 as soon as he seen a Ford Escort driven into the girders of an overpass one mile away!
I remember reading a book on tornados back in the 1960's and at that time, they thought the windspeed could reach 500 MPH.