You need a multimeter on the DC voltage setting (start at the highest range setting and work down if it is not auto-ranging).
You need to measure across these two white wires:
NOTE: even though the voltage across these two wires is likely to be very low (to drive the logic circuit) the whole thing could be floating at mains potential. This has the risk of killing you if you touch it. It is really not the sort of project you want to take on if you do not know what you are doing.
Having said that here’s how I control my dehumidifier, lots more buttons and LEDs than yours:
https://community.home-assistant.io/t/smartifying-a-dehumidifier-with-esphome/308994
Also this was written before the days of button entities.