I set up a Tor bridge a couple of days ago, but it sees hardly any traffic. How can I check that I did everything right and my bridge is actually registered in the Tor network? And can I find out if it's actually useful?
You can learn more about your bridge at the Globe service: http://globe.rndm.de/
For example, you can learn how much traffic your bridge pushed in the last days or weeks. You can also find out whether and how your bridge is given out to bridge users, like via email or https.
Also note that bridges that aren't given out are not useless. These bridges can be given out to censored users manually, which means they're less likely to be blocked. It makes sense to keep them running even if they're not pushing much traffic. If you prefer your Tor node to push much traffic, you should consider turning it into a non-exit relay. But bridges are important, too.
See my answer to a similar question here.
Briefly, search by nickname which you'll find on your ec2 instance in /etc/tor/torrc