If the captcha is coming from an environment under your control, you will likely need to implement some sort of method indicating you are in a test environment and have the captcha system return a known value or some indicator of what the expected value is.
If, on the other hand, the captcha is coming from another source out of your control, you are probably our of luck. At that point, you are essentially in the same boat as the spammers who are in a constant arms race to write software that can visually parse a captcha.
UPDATE
I feel the need to add some clarification to the ideas put forth in the question, answer and comments. Essentially you are dealing with one of the following situations (note that when I say 'your', I am referring to you, your company, client, etc):
1) Your form, Your captcha system: If this is the case, your best solution is to work with your developers to add a 'test' mode to your captchas, returning either a known value, or additional information in the page that indicates what the expected value should be. If you are able to make use of a tool, either written by you, or by another, that can successfully 'read' the captcha image, your system is broken. If you can do it in test mode, what is to stop anyone else (spammer, hacker, etc) from bypassing your captcha in exactly the same manner.
2) Your form, 3rd Party captcha system: If this is the case, your best solution is again to see if the system has some 'test' mode that you can make use of. I have no experiance with these systems myself but in general would guess that test methods exist for the major systems out there. A Google search of {Captcha System Name} automated testing should return some good hints as to how to go about testing with the system. If nothing good comes from that, your next bet would be to implement your own, internal, test only, dummy captcha system that works with some known value and make your captcha provider configurable so that you can point to your test system in test/dev/etc and your real system in production.
3) Another Form, Unknown captcha system: I am going to make a leap of faith here and assume this is not your case, but just for completeness I will include it. If this is your case, your not testing anything at all and are simply asking for help bypassing someone else's security mechanisms for your own reasons. If that is the case, please seek your assistance on less scrupulous sites.
