How do you know ink is cured on a shirt?
Wash it as you normally would launder a shirt and see if the ink comes off.
Get a washer and dryer at work or bring shirts home, that is the only way to be sure if the ink is cured. You can use all kinds of heat probes and temperature testing devices but there is no substitute for testing by a washer.
You can also get a rough idea by stretching the shirt gently against the grain and seeing if the ink cracks.
Also if you are not sure, then turn temperature up and/or belt speed down until the shirts barely scorch and then turn the temperature down or the belt up a bit. That may waste energy and is not ideal, but if you are unsure that is a good start.
Your customer is usually going to wash the shirts and they aren’t really wanting to hear about your temperature tapes, donut probes, pyrometers or non-contact infrared thermometer, they are hoping that you already tested the shirts by washing one.
You actually want to keep track of dryer temperature and belt speed and eventually figure out what works best, but the washer is the tool to find out if the ink is cured.