Maybe you could check their Alexa, Quantcast and Compete rankings then maybe youc an figure out their traffic
SEMRUSH or ALexa both are found in free FF plugin called Seoquake. They will give you an idea but ... I would say they are not exact. There are other methods. If you rank on the first page of a search engine for the terms they do then and say your at spot 5 them 1 there are many charts out there that will give you data on how much better one does over 5. Then again a well written description may add some bias too that click throughts to thier site.
I agree with reapr. The inaccuracy of the stats can often cause more harm than good. I remember changing an entire business five years ago on a whim caused by inaccurate stats ... don't make the same mistake!!
you can use this site: trafficestimate.com Note: the site you check need to have at least 2000-3000 visitors per day to show results... I personally tested the tool and it is pretty good...not 100% sure results...but still is good
All the above replies are good to get sites traffic information. But I think you might actually consider knowing their progress in SERP! In this case you need to analyze and focus on your competitors back-links!