How to recognise surfers connection
 

Hi any of you know how to recognise if the surfer is connected by a modem or something else?
I wanna recognise it to show the right ads ...
movie sites for surfers with hi-speed connection
pic sites and dialers for surfers with modem

Anyone know how to do that?

Thanks

Is there any way?

I've only one idea how to detect it... but It's not the best way (i hope that's not the only way)