The quickest way to determin threadfin from gizzards is to run your thumbnail down his "face" and if it catches his lower jaw and his mouth opens, the fish is a threadfin if you do not contact his mouth its a gizzard. Also the biggest difference is size. Gizzards get much bigger than threadfin. Blueback herring and Alewife's do not have as "deep" a body, they are more cigar shaped and the olny way to distinguish one form the other is to cut open their stomachs. The blueback will have a black lining in his stomach while the alewife will not.
"I knew my tippet was too big when the willow tree I'd hooked on my back cast came flying over my head." - Jimmy D Moore