This was 'O' Level Maths, when I was at school; I daresay it's GCSE Maths now. But in the Quiz League, to some people this was a classic either/or question – and in the Quiz League this is not seen as a good thing.
However ... there's an easy way to remember which is which. "Lowest common denominator" is a well–known phrase, meaning (in colloquial usage) something that is (to quote Wikipedia) "deliberately simplified so as to appeal to the largest possible number of people."
This should help us to remember that the denominator is the lowest (or lower, to be grammatically correct) part of the fraction. So the higher part – the number above the line – must be the numerator.
In mathematics, the lowest common denominator of a set of fractions is the lowest number that is a multiple of all their denominators. The Wikipedia page linked above has a fulsome summary of why the lowest common denominator is an important concept in arithmetic.
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