I have a Cisco modem - Can PRTG check its internet connection?

This can be done using the Cisco IP SLA sensor with the following configuration on the device itself. Simply execute the following commands on it:

Conf t
ip sla 1
  icmp-echo 8.8.8.8
  timeout 10000
  frequency 30
ip sla schedule schedule 1 life forever start-time now

A little information on the commands:

• icmp-echo 8.8.8.8
  This will ping the google DNS server.Enter anything you like to check connectivity
• timeout 10000
  If no reply within 10 seconds, this will trigger the PRTG sensor to goes down
• frequency 30
  This will generated ping every 30 seconds
• ip sla schedule schedule 1 life forever start-time now
  This is to enable IP SLA service, if not configured PRTG sensor will not work
  
  When this is done, simply create a new IP SLA sensor on the device and select the echo option. Now, when the address doesn't respond in a timely manner (10s in this case), the sensor will go down.
  
  
  Thanks to Samir Shaikh for sharing his solution! :)