To check the current mode, go to the magento installed folder using terminal then use the below command bin/magento deploy:mode:show

 [developer@user magento]$ bin/magento deploy:mode:show
Current application mode: default.

To change into developer mode, use the below command

[developer@user magento]$ bin/magento deploy:mode:set developer
Enabled developer mode.
[developer@user magento]$ bin/magento deploy:mode:show
Current application mode: developer.