Using dig to check DNS settings
You can use a browser-based DNS lookup tool, but you can also use the terminal to see the current DNS settings of your domain.
With the dig command you can see if there are any CNAME entries and where they are pointing to. Here we look up help.fortrabbit.com and see a CNAME pointing to the App URL like help-frbit.frb.io.
$ dig help.fortrabbit.com
;; Truncated, retrying in TCP mode.
; <<>> DiG 9.8.3-P1 <<>> ANY help.fortrabbit.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 44565
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;help.fortrabbit.com. IN ANY
;; ANSWER SECTION:
help.fortrabbit.com. 600 IN CNAME help-frbit.frb.io.
help-frbit.frb.io. 300 IN CNAME help-frbit.eu2.frbit.net.
help-frbit.eu2.frbit.net. 20 IN A 52.48.51.144
;; Query time: 863 msec
;; SERVER: 192.168.178.1#53(192.168.178.1)
;; WHEN: Wed Jun 1 10:25:40 2016
;; MSG SIZE rcvd: 122
Dig an IP
# This will print out the IP of your App
$ dig +short {{app-name}}.frb.io
See also here for why you'll probably need your App's IP.