Given that there doesn't appear to be a critical mass of legal professionals on this site, how do I know what answer on a question is correct? Votes don't reflect expert review.
A good answer should provide sources for its claims. You can read those sources and judge if the answer reflects them, and if they seem to be reliable.
A good rule of thumb is to assume that all answers posted are incorrect unless the answer proves itself to be correct.
What I primarily look for is if the answerer has the ability to explain details of an answer in a way that a layman cannot. I also look into any sources that an answerer provides, and make sure that the sources are credible, and that they confirm the answerer's conclusions.
To be completely honest and direct... you don't. When it comes right down to it, you're just reading posts from random people on the Internet. I think it's safe to assume good faith unless you have a specific reason not to — at least, I always do — but as you say, any particular answer might not come from someone with the most relevant expertise, or there may be an incredibly obscure special case involved, or an answer may have just become obsolete over time.
References and other supporting text included in answers, and votes, can be helpful, but there are no guarantees. For this reason, when I have questions and search the Internet and end up on Stack Exchange sites, I often find myself upvoting the questions but not the answers. In cases when I can test the answers out (as on Stack Overflow or Seasoned Advice, for example), I will return after a few minutes or hours (or in one case on Super User, years) to upvote where appropriate, but on a site like this that might not be possible.