I just had a test on this and this was the one problem I couldn't get:

lim as x -> infinity of (sqrt(x - 6) - sqrt(x))

I answered 0 but I wasn't really sure. I thought it may have been 6 or maybe infinity. The usual method I use is dividing everything by x raised to the highest power in the denominator then simplifying. But since the denominator is x^0 this didn't work...