What are some ways to find a leak in a pipe?
This is the easiest way I know of to find them:

1) Dry off all pipes with a paper towel

2) Run the water, garbage disposal, spray hose, etc. - anything you think contributes to the leak

3) Take a dry piece of paper towel and wipe each joint and pipe. Inspect the paper towel after each one.  When you find a spot thats wet, you've probably found your leak.