I presume the iphone reports it when transmitting but even if that wasn't the case there's plenty of stuff in the ip header (OS, browser type, etc...) to determ

I presume the iphone reports it when transmitting but even if that wasn't the case there's plenty of stuff in the ip header (OS, browser type, etc...) to determine if it's a phone or a computer on the other end. Plus PCs in paticular produce a lot of junk that would give it away. Stuff like IGMP calls would only come from a PC.