Even if you don't read the entire article above (though it's definitely worth a read if you're interested in the topic), definitely take a look at these tools to help with hashtag research:
Hashtagify.me
One of the most complete hashtag tools you will find, Hashtagify.me has reams of data you can use to analyze hashtags. The most helpful could very well be the first data you’re shown: related hashtags and their popularity. When you type in a hashtag, the service will show you other hashtags to consider and will display visually how popular each hashtag is and how closely it correlates to the original.
RiteTag
RiteTag helps ensure that the tags you use are well-chosen by showing you how good, great, or overused a particular hashtag is. The visual organization of hashtags into colored bars works great for quick analysis at-a-glance.
Tagboard
With Tagboard, you can see how your hashtag is used across multiple networks. The results pages on Tagboard show hashtagged posts from Twitter, Facebook, Instagram, Google+, Vine, and App.net
Twitalyzer
Though not an explicitly hashtag tool, Twitalyzer does show hashtags as part of its audit of Twitter accounts. Input the username of someone you want to investigate, and Twitalyzer can tell you what hashtags he or she uses most often. This can be really helpful in finding out how your niche’s influencers tweet.
Trendsmap
Local businesses might find value in Trendsmap, which shows you relevant hashtags that are being used in your geographic area.