Already during soldering it is good to make some test. In my case it was checking if all the diodes are done correctly (it wasn't in first try). To do so I used just small power supply and jumper wires:

Some more testing can be done when fully assembled and ESPBoard is populated with the ESP32 dev kit. How to upload and run ESPHome is well describe on their webpage so feel free to check out there: https://esphome.io/

For me it is enough to be able to change the relays from ESPHome webpage to be sure that system is working and all looks good. 

So not it needs to be put into the cases, mount on the DIN rail, program with some real ESPHome components and connected to homeassistant for further control.