To check either the other person is online or not you can guess by sending him/her a message. When other person in online and connected with Viber then the message you will send gets the double tick. Moreover, if the receiver reads your message then it will turn to a blue tick mark.
In case if the other person is offline then there will appear a single tick that indicates the other person is offline. If you make a call, if another person is online you will get the ringing status otherwise you will see the “calling” appear on your screen.
These are a few things that will help you to know that another person is online or not.