Taking their best guess is fine. Or just taking the top three comments that don't seem ridiculous.
In this case the top comment on /r/whatisthisthing is "Looks to be a perfect match for India’s PSLV third stage." with comparison photos, the second-top comment is "Looks like a fuel tank for a rocket of some type.", and the third-top comment is "looks very similar to the Delta IV heavy fuel tank".
It's a nice plausible answer and about as straightforward a question you can get for asking an expert.
In this case the top comment on /r/whatisthisthing is "Looks to be a perfect match for India’s PSLV third stage." with comparison photos, the second-top comment is "Looks like a fuel tank for a rocket of some type.", and the third-top comment is "looks very similar to the Delta IV heavy fuel tank".
It's a nice plausible answer and about as straightforward a question you can get for asking an expert.