Generally large spikes in upload data usage is the best indicator. Many consumer grade routers are starting to build in per-device bandwidth usage, which helps narrow down the culprit.
Invest in something like a cheap sonicwall. Without bunch of licenses, it can still show you all the traffic information and act as a very powerful firewall with all types of blocking. You can search "sonicwall soho".
If you don't want to invest, you can also use pfsense. You can use an old computer to turn it into a perfect firewall.
