"To appear" has a technical meaning in academia, though—it doesn't mean "I hope"; it means "it's been formally accepted but hasn't actually been put in 'print' yet."
That doesn't stop someone from lying about it, but it's not a casual claim, and doing so would probably bring community censure (as well as being easily falsifiable after time).
It is a bit more complicated, since this is a conference paper. Usually, if a conference paper is accepted, it is only published if the presentation was held (so if the speaker cancels, or doesnt show up, the publication is revoked).
Edit: All conference are different, I dont know if it applies to that one.
I have only ever attended one conference, but I attended it about 32 times, and the printed proceedings were in my hands before I attended any talks in the last dozen or so. How does revocation work in that event?
Well, it depends on the conference. I know this to be true for a certain IEEE conference, so I assumed it to be the same for this IEEE one, but I have to admit, I didnt check. You are right, I also remember the handouts at a different conference handed on a usb stick at arrival.
That doesn't stop someone from lying about it, but it's not a casual claim, and doing so would probably bring community censure (as well as being easily falsifiable after time).