It is tricky. Monitoring network traffic should be enough, the infected device has to connect to Command and Control, to receive orders and set up tunnels. It got harder after malware creators started using Telegram, Discord or even good old IRC for those purposes - it will be asy to notice only if they use a service no one else uses in your network.

Step 1: have you installed anything from an app store such as Google Play or Apple's app store?

Step 2: have you bought any devices made by large companies like LG or Samsung?