Few forwarders not sending data

Hey everyone, I have installed UF agents in 180 servers and i have seen the data coming to splunk yesterday. But now i have noticed three of them are not sending data, I mean i am seeing 177 hosts in splunk. So how can we find out what are those three UF's which is not sending data. I have configured all these using Deployment server. Same index and same sourcetype. And is there a way that we can get alerts when the forwarder stops sending data or gets any issue? I am using splunk 6.3. Thank you