First, whenever the current_user does any action, his updated_at will be set to Time.now:

# app/controllers/application_controller.rb
after_action :update_user_online, if: :user_signed_in?

private

def update_user_online
  current_user.try :touch
end

Next, we will just say that the user is online if he was updated_at within the last 2.minutes:

# app/models/user.rb
def online?
  updated_at > 2.minutes.ago
end

Now we can get true or false if we make a call like @user.online?:

# app/views/users/show.html.erb
<%= @user.online? %>