We can tell how long ago it died provided that:
The ratio of C12 to C14 at the moment of death is the same for every living thing, but the carbon-14 decays and is not replaced. The carbon-14 decays (with its half-life of 5,700 years) while the amount of C12 remains constant in the sample. So by comparing the ratio of C12 to C14, we can tell how long ago it died.
1. there has been no artificial change in the amount of C14 in the sample
2. the rate of addition of C14 to the atmosphere has remained constant
3. there has been no change in the rate of radioactive decay since the death of the organism being dated
4. there is still some C14 left to be measured
Of course it's not ridiculous. It's called a control. If you date something that is too old to give a result you should get no C14, and therefore an indication of a date of >50,000 (assuming 1-3 above). If you do get a result, then
The half-life is short, limiting its effectiveness to items that are about 50,000 yrs old or younger. Obviously, this short time frame certainly poses no problem for old earthrs since it would be ridiculous to use it to age something so far beyond its scope.
1. the sample is not as old as was thought
2. it has been contaminated
3. the dating method is invalid
The test is a control on the dating method. Creationists expect fossil wood and dinoaur bone to give a result, because we believe them to be 4500 years old. The test results bear that out.
Raciocarbon dating, like most things, is effective in certain situations and ineffective in others. Honest researchers know these limitations and avoid them, creationists know these limitations and purposely age things where the results are certain to be unreliable in the hopes of fooling their crowd into thinking the method is unreliable.
For example, radiocarbon dating is for dating organic things that should not be older than about 50,000 yrs. So if someone tries to date something like a 65 million year old dinosaur "bone" or millions of years old petrified "wood" they are either extremely stupid or they have an agenda to decieve others about the accuracy of this dating method. Its like claiming laptops do not work then proving it by throwing one in a lake and trying to use it. Laptops (and radiocarbon dating) work just fine in certain situations but not in others.
That assumes that tree ring data is fully valid. What creationists do is a test and the method fails under your assumptions; under YEC assumptions it is vindicated subject to a need to adjust for changes in the decay rate.
This dating method is calibrated by tree ring data and blind tested all the time. It has proven accurate and reliable.