Well that depends if you're talking about temperature or looks. :P If its temperature; "du er varm". If not, the younger people atleast would say "du er hot", since we use a lot of english words, but i think the closest norwegian word is "deilig". Be careful about that though, cause the term is a tad more vulgar than "hot" is i would say. It's usualy fine tho. You can use "vakker" for a man if you think he is, but I guess a lot of men would be offended, since it's usualy used as a "girly" term. The same "rules" apply for the norwegian word as the english one i guess.