How to verify GTM VS failover was successful?

What you're asking for is data about what answers were given in response to DNS queries. In GTM 11.5.x and later, there's logging you can enable logging on a listener, or record the decision log on a wide-ip basis, which will write data to the logs for every query coming in.

Failing that, you could create some sort of monitoring script on your network monitoring server, to periodically make a request to the GTM for that wideip, and record the result.

You could also look at the traffic patterns that hit the pool members themselves, though this would only be useful if the backup VS doesn't normally receive any traffic.