How could I check the domain nameservers change happened completely?

I have changed the nameservers for a domain and the transfer has
happened as I check the website the domain is now pointed to the newly
transferred hostname. However, there are still some instances where
the domain name is still going to the old nameservers.

Usually, it takes about 48 hours for the complete transfer to happen.
Is there a way to test and make sure that this has happened? Are
there some tools out there? If there is an alert/notification
mechanism that would kick out on a domain name that is even better.