How do I know by taste, if ham is bad. I have had pre cooked ham cooking in a slow cooker on warm for 3 days
I fully endorse the "when in doubt, throw it out" doctrine, although I personally wouldn't consider a sweet taste to be doubt. As rfusca wisely points out, you can't taste or smell several kinds of contamination, and the ones that you can taste or smell, are usually sour, bitter, or generally pungent.

I suggest you have a look at the following question:

Is it bad to leave the crock pot on “warm” (not low) all day?

If your crock pot has really been on the whole time, then it might be perfectly safe. You need to ascertain what the "warm" temperature is. Assuming you still have it running, just stick a thermometer in and see for yourself.

Anecdotally, I've heard reports of crock pot "warm" temperatures ranging from 150° F all the way to 220° F. Very unlikely that newer crock pots are anywhere near that high end, but anything above 140° F is cooking temperature and will kill bacteria rather than allowing them to breed. So if your thermometer test reads higher than that, it's probably still safe even after all this time. I can't speak for the quality of something that's been cooking for 3 straight days, but there's no accounting for taste...

On the other hand, if your thermometer reads anything lower than 140° F, then you should throw out what you have and stop doing this from now on. A truly "warm" but not "hot" temperature like 100° F is practically a giant incubator, even worse than just leaving it at room temperature.

P.S. You should read the manual for your particular crock pot, because even though it might be safe for the food, I'm not sure if it's safe for your pot. Many of them come with recommendations not to use the warm setting for more than 4 hours, and don't really explain why. Possibly, they think the temperature might not be enough to guarantee food safety, although another (to me more likely) possibility is that it can stain, corrode, or weaken the pot.