You can gets K type thermocouple porbes for sparkplugs all over the place. cheap ones are cheap because theyre thin wire and crappy metallurgy so they're not gonna read always right but for this stuff exact temps don't matter. everyone's just looking for that arbitrary number that they believe is an upper limit. This'd tell you if you're too hot. it's a great option for tuning, just like: hey how hot-ish do I get with this setup, but it's also definitely not a leave-it-on constant monitor the way a trailtech would be.