Have a look at the rim sides where the brake pads rub Isobel, to see if there's lots of wear and/or scoring. When new, there's a fine groove in the centre which is a wear indictor, and that should be the only deep score mark.
If you don't mind crawling around, peer up inside the rear of the housing on the chain side, ignore the small jockey wheel that tensions the chain, but look at the small motor sprocket just beyond it to see if the teeth have any "hooking", indicating lots of wear. You can lay the bike on it's side to see that.
Ask the owner to remove the battery as there's a yellow label on it which you can look at. It has date circles in it, and the original selling dealer should remove the one when he sold it, thus telling you the true age, of the battery at least. If it's not there, or the circle wasn't removed, ask to see the original receipt.
Otherwise, just look at the general condition and whether the chain is lubricated to see how well it's been looked after. The finish on these bikes is excellent and mine looks nearly new after nearly four years of hard use, so that one should look the same if it hasn't been abused.
