I have been using a D7000 for two years, and frequently in heavy rain/very humid conditions as I mostly use it for outdoor sports like mountaineering or sailing.
Does it really work in heavy rain, or is it only for few water drops?
Well, yes it does. Never had a problem using it for 30 minutes in heavy rain or on a sailing boat after it received a huge wave (I was so scared I dried it as fast as I could and removed the battery but it was fine).
That said - I am an engineer - having had a closer look at it I would not be so confident if you wanted to regularly shoot when it is raining. I feel more lucky than really safe with my still alive d7000 ;) I would certainly recommend a plastic bag.
How can I know which lenses are weather sealed too? Is there a list?
I can't provide anything more than Itai's answer on this part of the question.
Can I somehow make my curent lens "weather sealed" with some kind of plastic bag?
I prefer using something such as an Aquapac