If you enable forwarder monitoring on either the master or DMC it will provide you information on forwarders not reporting in or sending data. There are built in alerts for just this purpose, you just have to enable and configure them.
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This does exactly what you are looking for and a lot more too - it's very easy to use and very intuitive and the developer has some amazingly good documentation for a free application. I have just deployed this at a client and it works beautifully for tracking what index, sourcetypes and hosts have not sent data to Splunk - by default it will alert if data does not arrive for 1 hour, but that is all totally configurable.
NB: I have no connection to the app or the developer, I have just used the app.