Tricky one, we had this problem once, had to take the bannisters down to get the sofa in! It's not quite as easy as measuring the gap as I'm sure you've already thought of that, depends on how wide the hallway is and how high the stairs are. The only way to really be sure is try it.
Are you replacing a sofa with a bigger one??? if so was it hard to get the original sofa in? \if so you will have problems and be aware that many companies will not take back a sofa if it does not fit.