Technically, you cannot tell if someone is hacking your WiFi (trying to gain access to it). If they succeed, then the router should show their connection into the router either in the list of connected devices or in its log files, and also show increasing traffic over the ISP connection.
Note: once they are connected to your WiFi, they may start hacking any of your devices that are currently connected to your internal network either by WiFi or Ethernet. This traffic will not show up as data being used on your ISP connection.
You should ensure that your WiFi is using WPA2 (PSK) encryption with a random pass phrase of your own choosing (not the default one). If you must use words that can be found in a dictionary, pick at least three unrelated words (such as "monkey RaSpBeRrY CRIMSON") Use upper and lower case letters, you can also add numbers and punctuation. The WPA2 pass phrase can be between 8 and 63 characters in length.
You should also set your own password to access the router's configuration settings. Keep a separate record of this password. If you forget it the only way to recover is to reset the router to the factory state, which may require you to set up your personal settings again.
Be careful to whom you give out the WiFi pass phrase as they might pass it on.
I hope this helps.