Although I am still learning a lot from everyone here I usually start off on competitor research by asking my client who they think their competitors are. Then I perform searches for a range of their keywords and identify online competitors (who are often a different bunch to the clients).
Things that I look at include:
- Search Trends (Use Google Insights for keywords)
- Likihood Client + Competitors sites are to convert from users perspective
- Competitors Ranking URLs Page Authority (and Page Rank)
- Competitors Domain Authority
- What elements of onsite they are optimising
- The range of other keywords they are targeting
- Inbound Links (to domain and ranking URLs)
- Link Velocity (Majestic SEO is pretty damn good for this)
- Whether they have keywords in their URLs
- Anchor Text of inbound and internal links
- Internal Linking of sites
- Their Use of social media
This isnt an exhaustive list by any means and each time I run audits and competitive research I find new things that are (probably) affecting rankings.
The thing is, there is no way to cookie cut research as search engines themselves are evolving.