When using the chat feature in Gmail, it can be useful to know if contacts are using Android devices or computers.
Green Robot! comes in handy in determining that. To enable it in  :
Sign into Gmail, click on the gear icon and choose Settings.
Next, go to the Labs section and scroll down to Green Robot!
Enable it and at the bottom of the page  hit”Save Changes”.
The Gmail inbox will reload after this change is made. Simply, sign in to chat after that.
Gmail contacts using Android device will have a robot icon next to them (orange for idle, red for offline and green for available) while the rest will have same icons as before (dots and video camera icons).
This is a useful feature for Gmail chat.
Happy chatting.