I don't know of anything that specifically works via proxy, but you can use Wireshark
to inspect HTTP headers. Just use "http" as a filter to get all of your other traffic out of the log view.
UserAgent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:2.0b5pre) Gecko/20100830 Firefox/4.0b5pre
Laptop: HP Compaq nx6325 - Turion 64 X2 @ 2GHz, 2GB DDR2, 100GB HD, ATI Radeon X300, 15" LCD, Seven Pro
Handheld: Palm Treo 650 - Intel PXA270 @ 312MHz, 10MB RAM, 32MB flash, 2.7" LCD, Palm OS 5.4