Jeanne and la4angus are correct. The "thickness" term is readily misconstrued to incorporate the entire beef 'body' from neck to rump, and while that may appear to be a desirable trait - if the shoulder area is too wide (thick) the progeny from that type of animal will have a difficult time at birth, and the females with that phenotype characteristic will have a difficult time calving. "Smoothness and balance" are considered two very desirable traits along with "thickness" when seeking desirable phenotype traits for breeding stock particularly! As Jeanne said, "Fore-arm, rear leg, rump and loin" are the areas of concern. They are the "Money Cuts", and, granted, you should be able to "SEE" the muscling" in those areas, BUT - - when you are deciding on seedstock to purchase for your herd - - In My Opinion - - one should develop a 'touch' or 'feel', and be able to ascertain the difference between fat under the hide and more firm muscle tissue - along with the 'suppleness' of the hide. That may sound a little complex - but I know that it can be done.
DOC HARRIS