How to find competitor backlinks?
By Google or any website like Ahrefs?
Competitor link building is the best option to increase the backlinks of good type. The top 10 websites are your competitor's. But research and find the best competitor because a website which uses black hat technique also enter into top 10 sometimes. There may be little chances of collecting the spam links for your link building campaigns.
You can also use my own tool to find the competitors backlinks: http://www.metricbuzz.com/Backlink-Watch-Checker-Tool/
It results about 1000 links. Use those links and add similar to that kind to improve the rankings and SEO