Yup, official olympic ones are 20Kg. However, there are a lot of different onces around that are lighter.
As above, 20kg. But only if it's the 7-foot 2 inch oly bar. Shorter ones will weigh less, as will 1 inch bars. Also, most commercial gyms will have some kind of scales in, if you don't mind looking like a bit of a plonker for a few seconds.
Yeah usually 20kg, but the oly bars we use at my gym are 25kg...so i guess it could depend on the make etc too.