You can check the TTL using commands like dig and host. With dig, you can specify an external nameserver to check from.
dig @8.8.8.8+nocmd +noall +answer A domain.com
Using the host command, you can use something like this:
host -v -t a domain.com
There are many public nameservers you can query from. A few of them are listed below:
Google
8.8.8.8
8.8.4.4
Verizon
151.197.0.38
151.197.0.39
