WAVE toolbar gives you a very quick first impression about the quality of the currently browsed webpage's accessibility. At the press of a button, it provides visual cues by injecting icons and detailed information right into the page.
Download WAVE Evaluation Tool and install it.
Instead of installing it, you can use WAVE's online service on wave.webaim.org.
Activate WAVE by clicking its symbol in the browser toolbar.
You will then see icons appear directly on the page. Click on them to get more details.
The most interesting items are the red (errors) and yellow (alerts) ones.
In general, be careful with the displayed results. Automated test tools are very limited and tend to display both false-positive as well as false-negative results.