Logic problem

Here's one my favourite problems requiring simple logic. Only the brightest children I have worked with in Year 6 have ever been able to get this right!

There are four cards on a table. Each card has a letter on one side and a number on the other. The cards are laid out so that we can see two of the letters and two of the numbers:

A B 8 5

Someone makes this assertion: 'If a card has a vowel on one side then it has an even number on the other side.'

Which cards must you turn over to find out whether this assertion is true or not?