Most routers have a list that shows the clients who are currently connected to the network. It is wise to periodically review the connected devices and verify that all the devices connected are authorized. Some routers also send you notifications when a new device is connected.
Only allows specific devices to connect to your network. While every device that is permitted to connect will be assigned a MAC (Media Access Control) address, some hackers have found ways of mimicking these MAC addresses and therefore you ought to be watchful for that. Turning off guest networking is also recommended since it gives strangers access to your router thereby making your network unsecured.
It also helps to turn off the network when it is not in use. It is virtually impossible for a hacker to access the network when it is off.