Unless I'm mistaken, a cursory examination of the certificate supplied to your browser should show that it is one of those "mitm" root certs and not the certificate for the website you are browsing. That should make it simple to at least know your encrypted traffic is being hijacked.
A cursory examination of the certificate will appear to be exactly the same as the "real" one, with the exception of lengthy obscure keys and numbers that nobody would identify unless they compared them to the original side by side.
The proxy vendor MAY provide its own issuer description in lieu of faking the original, but it by no means has to. Fake certs can look just as authentic as real ones if your browser trusts the root cert that signed them.
Well yes, if you've got the same server and cert. Often the SSL frontend of big websites serve multiple certs so you'd have to have all of them to compare against.
Plugins like Convergence will verify who signed a cert for you, assuming the notaries aren't blocked by your corp proxy (and currently blocking any of them will kill the whole chain of trust, so it's not as useful as it seems).
In any case, your HTTP client having the right software and right root certs is the only way to know what is valid and what isn't. You could compare the fingerprint from your 3G phone's browser (or the issuers in the chain's fingerprints) to the cert from your PC's browser. Unless the phone is managed by your company too, in which case they can install the fake root cert there as well...
What's also kind of funny is it's easier to spoof a cert on a mobile phone than a PC. Phones can be updated over-the-air by the mobile provider while your PC has to give some kind of admin rights to your ISP. Since mobile devices are "the future of computing" this makes who controls the mobile device a scary proposition, especially in countries with oppressive regimes (or countries that like to shove internet legislation down your throat without asking).
