Along similar lines, i was watching mythbusters last night (probably a rerun i hadn't seen yet), and they tested the whole mice scares an elephant myth... Long story short, it turns out it's true! Elephants will go out of their way to avoid mice!
Mice I can handle. Large hairy spiders with many hairy legs, scuttling across your "equipment" while in an outhouse, *no* thank you
Back on topic: To test if your leak is from tank to bowl drop some food coloring in the tank and if it goes into the bowl then yep, it's the flapper.
I think this is true except if water is overflowing into the overflow pipe which is easy to check just by looking at the water level. If the problem is intermittent there could be debris in the incoming toilet valve assembly causing the water to not turn off completely and slowly fill to the point of overflowing into the pipe, the next flush moves the debris off the seat and it's okay again until the debris finds it way back onto the seat. The dye would tell you if the flapper was leaking if the water level is below the overflow pipe.
