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).
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.
If you want to verify a certificate is signed by a "real" trusted CA, use a program that traces the chain of trust against a list of CAs that ship with browsers. I have such a tool here: https://github.com/psypete/public-bin/blob/public-bin/src/ne...