When I'm saying super easy keyword:
1- A Forum in the first page " more preferably Top 5 "
2- Amazon / Big E-commerce sites ranking on a "Best xyz Keyword"
3- Pinterest, Flipboard etc in Page 1.
4- 1 or 2 small sites have the keyword in Title with shitty on page optimization.
5- I check the content for the first places if #1 have 300 words with no images and horrible on page optimization, then I made a content of 2000 words with images and videos and well optimized, people will stay more on my site so then It will outrank #1 with no doubt " If it's a small site "
These are some evidence that the keyword is easy.
I would say also you can reverse engineer the Forum point. I mean if you know that when forums rank in page 1 the keyword is easy, So if you know all keywords that this forum is ranking there will be a big probability to find a lot of other easy keywords.