Hi,
Welcome to the weird workd of Internet Muppetfandom.
There are quite a few sites on the web where you may find details of which puppeteers performed which characters. In the good old days (of The Muppet Show - or TMS as we call it) it was largely the custom for characters to be developed for specific performers (sometimes with traits of their puppeteers worked in). Anyway most characters usually had a regular performer.
Internet Muppet fans have spent many many hours over the years trying to track down information about who performed what in the various productions. We've trawled through film credits, watched videos assidously trying to spot voices or even puppetry styles, gone through as much background info as we can find, etc.
A few sites you will find some of this information on includes:
Chris and Sue's Unofficial Web Guide To Muppet People And Productions:
http://www.punchandjewelry.com/legacy/html/mpindex.htm The first major list of this sort of info on the web. Some of the minor entries may be inaccurate (because they weren't checked properly), but generally it is a very good source list.
The Internet Movie Database - not the most accurate of sources (as it is difficult to get errors corrected in it), but nethertheless quite good on main characters.
Muppet Wiki -
http://muppet.wikia.com/wiki/Main_Page - still a work in progress, with lots of effort made at accuracy where possible.
Kermitage:
http://www.kermitage.com/ - The official Henson fansite (if it's working). Being official it is a reliable source, even on more obscure characters.
Puppeteer Louise Gold's fansite:
http://www.qsulis.demon.co.uk/Website_Louise_Gold/Start.htm - well if you look at the Muppet section you will find quite a lot of details as to who performed what, not just on Ms Gold, but also often her Muppeteer collegues as well.
Finally you may find Craig Silverstein's old Muppet FAQ of some use:
http://www-cs-students.stanford.edu/~csilvers/muppet-faq.html - It isn't updated much any more (I guess Mr Silverstein's too busy with Google). But it's still good to read it.
Hope that helps
Emma