How can I check that a serial connection works?

Configuration
1

Got HA installed as a vm, and an Arduino Nano is connected over USB serial passthrough. I had it working for a couple of days, then I messed up changing the name in configuration.yaml:

sensor:
  - platform: serial
    name: Nano    <-

First error was Entitiy unavailable (of course). Now I inserted a new card with the right entity, but it comes back as Unknown instead.

Would be nice to see if HA can talk to the Arduino at all (it works with the host).