Sump Leak

[brainbug007]

I was in the garage the other day and noticed a small amount of oil on the floor under the engine. Had a closer look and the whole bottom of my sump is covered in a thin layer of oil and slowly dripping off onto the floor. It's really hard to work out where the leak is so I'm hoping someone knows a clever way to find it and seal it without having to take the whole engine/gearbox out of the car?

edit. when I orginally shortened & welded up the sump I left it for a week full of water to check for leaks and didn't find any. Does oil seep through things easier than water??