They should be bright, inquisative yet timid(depending on their human contact) and have have their ears pricked up watching you. Always look at where they've kept them, the conditions etc. Try and have a look at their mum too. She too should be bright, maybe slightly thin due to feeding piglets, but should still look healthy. If the seller is happy to show you round their ground/farm and its in good condition then usually the animals are kept in the same standard.
If in doubt, jyst say you'll think about it!!!