If you ever developing react native apps but don't know how to open debug mode in IOS/Android Simulators.
Here is the shortcuts:

• On IOS Simulator hover over the screen and press: CMD + D
• On Android Simulators hover over the screen and press: CMD + M

Extras:
Use console.log or react-native-debugger

• After downloading and opening of react-native-debugger press CDM + T to connect to your app at specific port (ex: 19001)
• Then go any simulator and again press CMD + D or CMD + M and choose debug remote JS option.