The best and most reliable way, if you're familiar with networking, you can use
Wireshark and look for suspicious requests.
You can also look at a simple traceroute printout between you and the internet, if you see hops that shouldn't be there then it's possible you're being monitored.
Additionally, by simply pinging all the local machines (nmap -sP 192.168.1.0/24 will do this quickly) and then checking the ARP table (arp -an) for duplicates, you can detect
ARP poisoning quite quickly.
However, if you're unfamiliar with networking, you can use the below mentioned tool to detect if someone is monitoring your network traffics, and it also provides security by alerting or stopping attacks:
- XArp: Advanced ARP spoofing detection, active probing and passive checks. Two user interfaces: normal view with predefined security levels, pro view with per-interface configuration of detection modules and active validation. Windows and Linux, GUI-based.
Bonus: To avoid being monitored over Wi-Fi networks,
1. Use over-the-top encryption products that encrypt data prior to transmission off your device.
2. Use a VPN solutions to establish an encrypted tunnel.
Here's a video on
How to detect ARP Spoofing attack in Windows 10
Hope it helps.


And let me know if anything is unclear, or if there is something you would like more detail.