what i do is check the page rank of the first page and if its is above a 4 then I dont ever touch it. I have lost lots of battles!
try checking the keyword density and research for tools that could simply give you the match of the keyword.
there's no thing such as good or bad keywords..As an SEO its depends on you on how you will make those keyword rank.
Your keyword is good or no it may be checked from Adword but its depends on content of web page.For good result of keyword the contents of web pages should be done according to keywords.
use free google adwords tool for keywords and search high density keywords, this is good and easy method to search good keywords.
Its all depend on ur experience in seo and keyword analysis and research skills. Most important and common thing is ur keywords must be related to ur product and service ur offering.