Where to get second opinions on independent research? Advisor mentorship, department peers, passing extended abstracts around at conferences etc. all come to mind for an affiliated researcher. None of those options except the last one apply to someone without an affiliation and the last one is cost prohibitive most of the time. What are the options for someone without access to traditional options like these?
Assuming the research is at an advanced enough stage, the best option is certainly to submit a paper to a peer-reviewed journal: free independent expert opinions, provided with advice on how to improve it!



[edit] If the research is at an early stage, there are often venues which welcome submissions of prospective ideas and "works in progress": it's often the case for small-scale specialized workshops, but there might also be journals which fit this profile as well (this probably depends on the field).

Of course it's always possible to contact academics to propose a collaboration with them. Some might be interested, but if you're asking them for guidance and mentoring then why not do a PhD with them? That's the standard way to get adequate research training.