Remove the bolt carrier from the upper. Remove the bolt from the carrier. Remove the ejector. Remove the extractor. Replace the ejectorless/extractorless bolt into the carrier. Insert the carrier back into the upper. Insert the Go gauge in the chamber. Gently push the carrier forward, it should close and lock on the Go gauge. Remove the Go gauge from the chamber. Insert the No Go gauge in the chamber. Gently push the carrier forward, it should not close and lock on the No Go gauge.
Assuming you have .223 headspace gauges. 5.56 military gauges are different. See the tacked headspace thread above for the details.
ETA your chamber, extension, bolt face and gauges should be squeaky clean for an accurate reading. We are talking about .001" or .0001" measurements (depending upon which gauges you have).