Victor's 1950 advice is good. I have tried variations of his technique many times. It can be tedious but you keep disconnecting electrical components until you get no spark.
Another ways to test for current flow instead of looking for a spark is to disconnect both battery cables and put a continuity tester between the positive and negative cables. Then, keep disconnecting leads to devices until the continuity light goes out.
Once I had a Volkswagen squareback that was draining batteries. After I was told the generator was bad (wrong), I was told the voltage regulator was bad (wrong) and finally I was told the battery was bad (wrong again). Luckily I was dealing with a parts supplier who was fair and honest (and embaressed). One of counter men said to do the continuity tester diagnosis. After several hours of disconnecting and reconnecting many electrical components, I found the problem in a non-working clock (since it was not working, I did not test it until last). The reason it was not working was that it had shorted out internally and was draining electricity. The short was apparently minor at first (I bought the vehicle 3 years before and the clock was not working) but as time went on, the short got worse and drained more current. The inside of the clock was charred and would most likely have eventually caused a fire if not found.
By the way, if you have an electric clock, disconnect it first (it draws a small amount of current at all times).
Good luck,
Tim
