Adding something to the point about being specific:
The example of "how do these hands look in this one panel" makes it seem like you can only be specific about a tiny portion of your comic, such as a single panel or perhaps a single page. This is not true. You can ask specific questions about a large portion, or even the entirety, of a long series.
Example:
"My finished story has a twist ending, and I've placed some breadcrumbs hinting at it along the way in [links to specific pages]. But I'm worried that maybe the hints were too subtle, making the ending seem like it came out of nowhere. What do you think?"
This is an issue that spans over a large portion of a story, but the question is still specific.
HOWEVER... even though the questions are specific, reading a long series is very time consuming. Therefore it's still harder to get feedback of this nature, compared to a quick drive-by "hey how is this one panel" critique. But it's still something to think about!