If you can implement 301 redirect, you should do it, I think its better way.
Otherwise you can use canonical tag on pages if you have any technical difficulty to implement 301 redirect.
There are many online tools that you can use to check back links like backlinkwatch and ahrefs, some of them might be paid. If you are looking for free then you can try openlinkprofiler.
I think guest posting is the most effective method to get quality back links from websites/blogs in similar niche. Article submission, forum posting and blog commenting are still working well in 2016.