Just a few sites that let you very easily check on your DNS records:
whatsmydns.net: this one doesn’t give a lot of details (very few, actually, only the resolving IP of a specific domain or subdomain), but gives the results as seen from several (at the moment 20) locations all around the world. So this is great to see if your DNS record is properly propagated everywhere.
intoDNS.com and Pingability.com: on the contrary, those run from only one location (at least one at a time), but they perform a deep analysis of all your DNS record, check for inadequate settings, and offer advice to improve those, if needed. So this is great to diagnose a misconfigured DNS record, once you’ve made sure it propagated. Pingability even does some checking to make sure your e-mail server isn’t configured as an open relay.
I had forgotten about this post, so when I did my latest migration last week I found a couple of new ones:
Zonemaster: kind of similar to intoDNS in my opinion, except that as far as I know it’s the only one in my list that will check both TCP and UDP on your DNS server. So it’s the only one that caught that my current DNS server is actually not reachable via TCP (maybe this explains why I couldn’t get AXFR requests to work on this one, while it worked fine with a seemingly identical configuration on my previous server). It also is the only one in my list that does test AXFR requests directly.
DNS Health @ Pingdom: just another one, which doesn’t give much details and keep its results in a cache you can’t empty, so you can’t repeat the test quickly to test your modifications. However, a noticeable thing about it is that you can use it to test an undelegated domain name.