C gets really obvious red eyes when he gets tired. As soon as they start looking a little pink, I change his diaper, feed him and try to get him to sleep.
I look for sleep cues and then put him down again right away. DS rubs his eyes when he gets tired. Yesterday, his morning nap was a catnap (20 mins) and he started rubbing his eyes about 1 1/2 hours later. So back to bed he went, and slept for an hour (his normal nap length).