I just used to see what naps we got each day and if one was short I would offer an EBT. Mine often didn't/ don't yawn until I am zipping up the sleeping bag but will have other signs. Things like rubbing their hair/head, pulling their ears, being grizzly or laying their heads on me, red or watery eyes or red cheeks and asking to nurse are all regular signals I need to get them to sleep ASAP.