How can you tell if an old frame was originally built to take 27 inch wheels or 700s?

there is only going to be 4mm difference ?! 622mm vs 630mm diameter therefore the wheel will be closer/further by 4mm...

hardly noticable... really.

Dont forget they most commonly take 27 1 1/4" tyres, which makes the overall wheel diameter quite a bit bigger than the 23-25c on 700c.