There are many tools available for checking competitor site's traffic. Google webmaster tool also give the idea about it.
I have a website, it was born after its competitors 3 years. But now it passed over all and get on top of google! I dont care about my competitors. Just do my best! Now my competitors try to attack my site everyday!
Yes you can check only competitors back link not traffic. If do you want to check any competitors back links then i want to suggest you some useful tools http://www.majesticseo.com/ http://www.backlinkwatch.com/ http://blekko.com/ http://www.opensiteexplorer.org/ They are very useful and popular competitors back links check tools
There are lots of online traffic estimators available on internet which helps in analyzing the traffic stats. Below mentioned are very good sites to check competitor’s traffic. alexa.com compete.com quantcast.com websitetrafficrankings.com
Alexa Rank can only be reliable measure on sites with Alexa Rank<100K otherwise is rather easy to game Alexa.
HI Alexa and quantcast is best tool to find out traffic and source of traffic of your competitors. Quantcast provide you specific area, gender , occupation and addicted user of that website. And alexa also offer you the keyword used by competitors and form where they are getting more traffic.
According to me if you find out how much traffic your competitor's site have, then use google webmaster tool and opensiteexplore tool.