SEMRush is the tool that I use to find competitors keywords that are driving traffic. Alexa and Open Site Explorer also gives a few for free. Another way (not necessarily as easy), is to visit the competitors website. Go to pages that you believe they are ranking well for. Look at the title of their page, along with H1 tags to manually find keywords. You can then take those keywords, and throw them into a ranking tool (or just manually look them up in incognito mode).