nowadays I mostly check to see if google answers the keyword directly. This scumbag maneuver takes a lot of traffic from our mouths, so if I see the answer showing up directly I usually (not always) avoid the keyword, which sucks because I had about 400 long tail keywords that I thought were amazing in a niche that I'm researching, only to look up about 250 of them directly in google and see that they're already answered by google themselves. Phucking rats...
And I'm talking about REALLY long tail (12-14 words long) keywords, even these are answered directly by google now...
Aside from this, I also check what's ranking on the first page of google. I feel like "ranking in top 10" will soon be an outdated phrase because for many queries there's no top 10 anymore because the 1st page is full of PAAs, ads, and youtube, which basically cuts the traffic by 90%, which makes SEO obsolete for those keywords in my non-expert opinion.
So anyway - assuming that google doesn't answer the keyword directly and that there is at least 1 organic spot left "above the fold" that I can aim for - the next thing that I look for is the type and quality of sites that are ranking on the 1st page... or whatever's left of it anyway.
If those sites seem approachable (by "approachable" I usually mean how their design and content looks for the keyword I'm targeting) I mark the keyword as good.