I think as you state on your profile you don't see Asians then you should tell them on the phone when they try to take the booking.
Incidentally if they sound nice and polite to you and pass your screening then why does it matter if they are Asian? I find timewasters and idiots give themselves away when making the booking and have had many lovely reliable Asian clients from India, Pakistan, Bangladesh, Malaysia, Hong Kong, The Philippines and Japan and so on. It seems a huge number of men in the world to rule out.
What happens if you don't ask and they show and are Asian, do you turn them away? I know plenty of Indian men who have or use a western name.