Look at the headers. I don't know if they actually say Turbo somewhere (though I could check when I get home, there are several websites that will show you your own headers), but I do know they clearly identify themselves as being for a proxy of some kind.
Though alternatively you could use HTTPS, Turbo does not handle any secure traffic.
Mind you, there's no comparable answer for people using Opera Mini on a mobile device. While it is easy to identify Mini users, Mini always uses the proxy. You should try to detect browsers that won't support your site and give those people something useful. Some of the devices that Mini runs on can't support a full browser; you may not be able to tell them to use something else - at least, until they can find a real computer to use.