Display the current mode in Magento 2 using CLI/Command Prompt
bin/magento deploy:mode:show
This will result in something like below
Current application mode: {mode}.
Set/Change mode in Magento 2 using CLI/Command Prompt
bin/magento deploy:mode:set {mode}
{mode} can be either default, developer, or production. If changing to developer mode, out put will be like below