How can you tell/see?
In my experience anything you render with render detail 0.7 and GI 2 isn't extremely different from using higher GI settings.
Higher GI settings than 2 looks better if you consider integration of objects into the scene, for example, but the effect is very subtle.
However, I can show you 3 versions of my Dutch Landscape (or pretty much any other) scene with GI 2, GI 3 and GI 4 and I'm pretty sure you can't tell which of the 3 uses the highest GI settings. Oshyan and I couldn't at least. It's so subtle
I know this one took quite a while to render so you're probably right Richard, quite sure of that actually, but I just don't think you can eyeball it from multiple images, let alone a single one.