How can I check if a DNS server is failing to respond?

I use stubby for my DNS over TLS resolution. I rotate through several different servers setup in my config file, “stubby.yml”. Recently it appears the some resolutions take longer than others. I suspect one of the servers maybe off line or slowing down. Is there anyway in the logs or somewhere I can see a DNS request timeout? My only other option is to try each server individually.

Not that I can find.

If you want to investigate, the tool for it is dig.
(Also very useful to test if DNS caching is working or not - which it doesn't by default on Ubuntu).

Thanks. I couldn't find the right DIG command. It seemed I could test each server individually, but it was not clear if it was a TLS connection or not. So I slowly started to remove various servers from my yml file. I don't know if it is a coincidence or not, but after removing the NextDNS entry, everything sped up.