The most obvious type of monitoring you'll want to perform is Flume agent process monitoring, that is, making sure the agent is still running. There are many products that do this kind of process monitoring, so there is no way we can cover them all. If you work at a company of any reasonable size, chances are there is already a system in place for this. If this is the case, do not go off and build your own. The last thing operations wants is yet another screen to watch 24/7.
If you do not already have something in place, one freemium option is Monit (http://mmonit.com/monit/). The developers of Monit have a paid version that provides more bells and whistles you may want to consider. Even in the free form, it can provide you with a way to check whether the Flume agent is running, restart it if it isn't, and send you an e-mail when this happens so that you can look into why it died.
Monit does much more, but this functionality is what we will cover here. If...
