Check which kind of test they'll ask you to do and practice it on line, using Google to find other places where they can offer some trial on line (I found them on University web sites and on the brand site of the test). I am not sure if they'll ask you to repeat it in front of them or not, but my feeling was that it was just a required formality; however if you practice it in advance, it won't matter, you can do it twice or trice.