I would recommend that you install the browser plugin
http://project-gc.com/Home/UserScript
That will display checkers on the cache page and it is easy do see if one exist
Else you can search for the GC code here
http://project-gc.com/Tools/Challenges to look if a checker already exist. It will also be displayed on the maps with pass/fail icon.
The last page to search is this support forum look for the gc code and you may found old questions and responds why i checker is impossible.
If you don't find a checker and don't find a reason that is is impossible feel free to ask for one.
use the following line as the question
Checker request: GC code challeng name
ex
Checker request: GC4EH58 Die Zwillings Challenge
and a link to the challenge in the text like
http://coord.info/GC4EH58
It can be copied fromt the down arrow right of the gc code on the cache page is prefererad.
And include a short description of the challenge so it is easy to se what challenge it is and if the will be many comments it is easy to recall the checker.
Look at the link lik below it is a good request but i would change the link to the short coord.info link.
http://project-gc.com/qa/?qa=7951/checker-request-gc4eh58-die-zwillings-challenge
Don't forget to set the category checker request when you ask the question
in the tags filed start to write challenge and select the "challenge-checker" alternative.
You will probably have a quick answer if it can be tagged directly or if it is impossible to create.
If it is possible but need a new script it is quite likely that you done get a respons.
If not script writer feel like writing a checker there is often no response.
I often write a commen if it is possible but is is hard to write and therefore not likely to be created