Can't say I ever had to worry about it. In the sink or in the drain? In the sink you could try a cup or something and if it;s in the drain you would probably have to take it apart. Do you have a shop-vac? J/K.
This just happened to me, I folded a piece of tin foil and coated it in peanut butter then I put one end down the drain while the other end was still in sink with something heavy on it so it did not go down the sink as well. Left it for 10minutes and when I went and checked Micky was siting in the sink :-) First thing I did was cover the plug hole before I tried to get him out!
I really have no idea, other than taking the pipes of the sink apart. I am sorry that this has happened. If you have a landlord, you should call and see if they can do it, or will allow you to do it. Hope this helps you out.
Oh My Goodness. Place a length of cloth, or better yet a thin branch down the pipe so it can climb out. Or remove the trap, so it can fall out. How did it get in the sink to begin with. Please let this turn out well.