Asking a professor whether my application has been accepted or not
When a department decides to accept someone, they try to tell them as promptly as they reasonably can (sometimes they need to get administrative approval or the like), so delay indicates that a final decision has not been made.  If you haven't heard anything, you can assume you have not been accepted yet.  It's theoretically possible that the acceptance e-mail got caught in a spam filter, but that's probably not likely enough to be worth worrying about.  

If you reach the point where large numbers of people have heard acceptances and rejections, but you have heard nothing, then it's worth asking about your status, just to make sure there wasn't a communication difficulty.  It's also worthwhile if you are facing an upcoming deadline and hope for an answer before that point.

However, if just a few students have received acceptances so far, then your best option is to wait and see what happens.  Asking for a status update will likely just lead to a response along the lines of "Sorry, we haven't made a final decision yet.  We'll let you know when we do."  (And asking more than once is likely to annoy the committee.  It's better to save your request in case it matters more later.)