Checking the WHOIS record for your domain name is a good way of confirming that your nameserver change has completed successfully at the registry. However, WHOIS records are not directly related to DNS propagation. At most, a change in the WHOIS record confirms the root nameservers should now answer with your new nameservers. The TTL value is still in effect and DNS resolvers around the world may only ask the root nameservers for any updates once the original TTL has expired.