I think aspiring rationalists especially will be looking for signs of growth and that perhaps without those signs they will become skeptical of the value of meditation.

Good point. One benefit of taking a long retreat is that (at least in the case of the dhamma.org courses) you will probably either have a number of insights or experience a significantly different state of mind. Either of these would be evidence warranting further investigation into meditation.

I'll try to think of signs of growth for those learning in the battleground of real life.