How do I know it’s an Emperor? Fear not, in the vast majority of cases you know straight away. The power of flight, together with size and sheer class are unmistakable. They fly differently to the White Admiral, so you will not mistake iris for camilla. If in doubt, it’s not iris. Silver-washed Fritillaries seen high up in silhouette are difficult, being of a similar size, but they fly very differently and are of the wrong shape.
You can mistake iris at a distance for a high flying Red Admiral at times, though the latter is smaller and the red is almost invariably apparent. Painted Ladies also can fly similarly, but they are smaller and much paler. Ditto the Comma. Anyone mistaking Purple Emperor for Purple Hairstreak should give up butterflying altogether and take up ironing.
Purple Emperors have two modes of flight. First, in windy conditions and when chasing, they use a powerful rapid wing beat mode, with wings rising up and down through a 150 degree spectrum. In calm conditions they skim, often at speed, with hardly a wing beat. The females tend to use the latter mode most, though they can certainly put on speed when they want to.
Large butterflies seen around the tree tops circling each other 2-3 times before whizzing off out of sight and out of mind in a high speed chase are male iris (or male iris beating up male cardui or male atalanta).