This is how I do it:
Check the number of results in quotes "keyword".
Check the number of backlinks the top 5 domains have.
Check the number of unique domains that contribute to the total link profile. If a blog has 5k backlinks from 5 domains, that makes things easier for you. But if a blog has 1k backlinks from 500k domains that is comparatively difficult.
Check the domain age.
I also check for web 2.0s ranking for the keyword. If there are a few web 2.0s ranking in the first results, it is usually a sign that the keyword might be easy to rank for.
How do I find out if I can go after the keyword/how much work it would take?
The answer to this question depends on how good your skills are, there is no universal measure for this.
What I do is measure a new niche in comparison with my last successful SEO campaign.
Let's say I ranked #1 for "dummy keyword" and it took me X hours and Y amount of content and Z number of backlinks.
If the metrics (backlinks/da/pa/domain age etc) for the new niche is twice as much as my previous campaign, it would be safe to assume it would easily take more than twice the work I put into my last campaign.
Developing "the knack"
After you have analyzed shit tons of keywords, ranked a bunch of them, you develop a knack for gauging competition. Now a days, after I have the metrics in front of me I know (roughly) how much time/money/content/backlinks etc it will take me to rank in the top spots without too much calculations. Keep working, and you will get the hang of it too.
Staying In The Game
SEO can be slow at times, especially if you are going after high comp keywords. If you are just starting out, I would suggest you go ahead and go after the low hanging fruits, the keywords that are very easy to rank for. They might not bring you a lot of money, but they will give you clarity as to what is doable at your level. This advice might not be relevant to everyone, but when I was starting out in 2009 I made the foolish mistake of going after keywords that were beyond my league at that time. I wasted a lot of time, energy and resource and then utterly fail. If I had gone after the easy keywords, I would have gained clarity about which keywords I can and should go after. Brush up your keyword research skills.