It can be hard to tell the truth from the lies, but I disagree with lil b on the point of people with illnesses not making threads about it.
I would think that if the person just found out about their illness, they might make a serious thread in the GDF explaining their situation, so to let people know why they might not be themselves, or for their lack of being on KMC. Also the internet could be the easiest way to deal with the illness, by talking with good friends that you find on here, you can cope with it. I know that when I had a health problem I was really worried about I told a few choice friends via IM, and they helped me with it. Sure I didn't make a thread about it, ut it's the same deal.
You never know though, some people out there are just looking for some cheap attention from people they will probably never meet. I myself do not lie to people about who I am, or what I look like, or how sick I am, because I hope to meet some people on KMC, and for them to meet someone completely different than what they expected will prove the lie. Then you wouldnt have that person as a friend anymore, and they could tell someone else, etc.
It's all about ethics, some have great ethic standpoints, while others...well, they have none.
Just believe the person until a flaw in their story comes up. Then you will know who is the liar and who is truly sick.