Noticed I had some trans fluid pooling in the bilge after a recent trip.   Fluid level is still not at the low level mark so it must have just started.

I can't seem to figure out where the leak is coming from.   Whats the best way to track it down? 

I laid some brown paper in the bilge under the transmission for a couple days but nothing dripped on it.  This might be a dumb question but do I need the engine cranked to be able to see the leak?   

thanks for any input.

It'll leak into the bellhousing and from there eventually into the bilge

Whatever is leaking, it'll leak more when the transmission is operating.

A few other choices would be the rear seal, the cooling hoses and fittings where they screw into the transmission and the oil cooler, the cooler itself, the gasket where the Forward and Reverse adapter mounts to the case, 

There are some other spots, but if you run it with the brown paper under it, you should be able to narrow it down.

Maybe you'll get lucky and it'll be a hose or fitting