Ask for details. The leaders have rehearsed top-level answers and can express them confidently and fluently. But we need to know if there is anything underneath the surface, beyond the well-crafted lines.
Take Afghanistan. They will all say that the fight is important, that a well-resourced military effort is key but will not be enough and that a politico-economic strategy is needed. Gordon Brown will try to sound presidential, talking about ordering troops here and there and giving the kit the generals “on the ground” have asked for. David Cameron and Nick Clegg will land a few equipment-related punches and then talk about the need to engage militants politically, inducing them to leave the insurgency.
Clegg will hint that his support for the war is conditional and perhaps even time-bound. Cameron may raise his mention for Paddy Ashdown's scuppered appointment as the UN's man in Kabul - something Brown did not back to the hilt and, which shows that Tory leader gets on with key Lib Dems.
But we need to get into the weeds. You should explore what they think can be done to make negotiations work, what should be done with Hamid Karzai (and his family), whether UK troops should be willing to depart Helmand for Kandahar, whether its time for the UK (like the US, Holland, Canada and Denmark) to set deadlines, however soft, for troop withdrawal, and how to deal with Pakistan. Details, details.
Paddy Ashdown used to say “big thumbs on little maps hurts the chaps”. How true. Enough “big thumb” approaches to British foreign policy. Details are required. Ask for details.