hebe donna addenda

hello. ive got 2 hebes; donna addenda britta and nikka. stupidly I didn't protect them over winter and we had some minus temperatures. how can I tell if they're dead? they certainly look dead but I don't want to chuck 'em if there's still hope, especially nikka who just flowered and flowered all last year and was covered in bees. I really coveted the britta because of the dark purple blue flowers yet it was nikka that actually shone out in the garden, the colour is just gorgeous and its not available to buy anywhere. any advice would be most welcome.