There are certain fonts that have a "caption" option. But like you, I am not seeing a consistent rule. Perhaps a web search might provide some info.
In the meantime, I will share with you my own default rules if that helps. Here is how I make my captions stand out:
1. Since most of the time black font is used, I tend to go with a gray color for the caption (if colored font, then a different color shade of used color, or different color if appropriate with design)
2. I often italicize when possible and it seems visually pleasing
3. I take the font size down a pt or 2 depending on space, design, font choice etc.
That is my take on captions! I hope this is what you were looking for.
SSU EdEon - Scientific Illustrator
I'm not arguing, I'm just explaining why I am right :)