There's no way you would know based on the confirmation the cruise line or your agent provides you. They can see it on their confirmation and tell you, but you can't see it on yours.
However, you can go to the cruise line's website, check out the price for one person and then check out the price for two people. (The price you'll be looking at will include the NCF's but not taxes.) If you see the difference is quite a bit, then chances are they're only charging it once. If there's a minimal difference, then chance are they are charging more.
I know for a fact that NCL charges double the NCF's. However, here's a little trick they play; if you book one of the new single cabins, like on the Epic, you'll only pay the NCF's once. But if you book any other cabin as a single on the Epic, you'll pay double the NCF's!
As I mentioned, NCF's are basically the total of the port fees. Ports charge those fees based on the number of people on the ship, not the number of berths and not double for a single person in a double cabin. So when the cruise line charges more than once for the NCF's, it's just their way of taking advantage of the situation to make extra profit. And that, my friends, is wrong!
And since we're on the subject of NCF's, years ago they were not included in the advertised rates. So people would see a rate for a cruise and think it was really great - until, of course, they went to book it and then the cruise line would add the NCF's and the taxes. Finally, after all the complaints, Congress took action and now requires all advertisement and quotes to include the NCF's in the total price.
However, nothing was in the law about what could or could not be included in the NCF's. Remember, NCF stands for non-commissionable fare.
So, as was the case last year, cruise lines will increase the NCF's even though the ports have not increased their fees. And none of the cruise lines will tell anyone what makes up those NCF's or how they came about the figures. When asked, they'll tell you it's an 'average' amount. Funny thing is, two ships from two different cruise lines going to the same ports will sometimes have a different NCF figure.
As with charging double the NCF's for singles when they should not be, often increasing the NCF's is just their way to increase the cost of the cruise without increase the amount of commission they pay to travel agents. And that, my friends, is also wrong.
But hey, they do it because they can get away with it.