There are some ways to monitor your masternode.
Use TomoWallet, just need to import your masternode owner address to tomowallet, you can receive the notification if your node is slashed, you also can see the reward report.
Use TomoMaster API, see the docs https://apidocs.tomochain.com/#masternode-checking, if return “1” OK, return “0” slashed masternode or proposed.
Get node status from RPC API https://apidocs.tomochain.com/#getcandidatestatus
Write a script to monitor if node does not sync full data by checking the latest block number of the node, It will be potential to get slashed. See the example: https://github.com/tomochain/e2e/blob/b0113237207f88d96e002ecdd66943ddd52224df/test/rpc.js#L41, the script the if the latest block number of the node is late 120 seconds with the latest blocknumber of the networks.
Share me if you have the other ways. Thank you.