Ah, now you're asking interesting questions. Your (maximum) speed should be somewhere in the contract/paperwork you received from Sky when you signed up. I'm pretty sure they'll tell you if you phone them. However: THAT'S NOT GUARANTEED! Unless you fork out a lot of money, you will only get contended bandwidth, meaning you share a certain amount of bandwidth with up to 50 of your neighbours. If Johnny Bloggs 3 houses down the road decides he wants to see an HD video from netflix, and some kids play online games with massive graphics, then that impacts everybody else. Especially as their movies usually get priority over your traffic (because interruptions in a movie or a game are much more detrimental than they are in other web traffic).
You can check your speed with any of a whole range of test sites (just type "check connection speed" into google). Note: this will vary depending on time of day, neighbours and some other factors.
There's more to this, but I shan't bore you all to death with the technical details.
