For legal reasons, most employers will tell interviewers not to reveal any information at all about how you did. Unfortunately, the US is a highly litigious society, with lots and lots of lawyers egging people to sue over just about anything....
As an interviewer, to indicate things are going well, you can smile, nod, give happy words of encouragement, get excited, etc.
When things are going poorly, you can frown, say "not quite", give sour words of encouragement, linger on the same unsolved problem, etc.
While HR / Legal usually have policies prohibiting direct feedback ("You're a 5 of 10 and we have 30 candidates who are all better than you. Also you went to Yale and that's lame."), there are many ways to provide feedback to the interview candidate without opening the company up to legal trouble. There are no laws against being human during an interview.
