if i were looking for inclusions with a naked eye...

[Dancing Fire]

under what lighting condition would be the easiest way to see the inclusion?

 

[moremoremore]

This is a dangerous question....

But I would say bathroom vanity (overhead) lights...that's where I cam face to face with the beast...and lost...so the stone got returned....

....but I'm getting a better one...

 

[valeria101]

Diffuse lighting... (sheet of paper in front of strong source, or facing a white wall instead of light source). "Diffuse" does not need to mean "week".

And lookup "darkfield illumination" - you can probably improvise the same somehow.