Frankly, online marketing decisions, the content from the online writer's pen and behavioural targeting all depend on those keywords we choose. My mother used to say, "Choose your words with care. Make them sweet because you never know when you'll have to eat them."
Same truth today.
Having said all that, here are some of the new tools of the trade:
Facebook lexicon - search how often a keyword is used on the walls of the world's largest social network.
Google Trends - search how often a word or phrase is searched for in Google or see how often it is used in news articles indexed by Google.
And as always, the spikes and dips tell the bigger story.