DS has very obvious tired signs. He does this face that looks like he's really concerned and kind of groans. Then he rubs his eyes. Very easy to read! As soon as he does this I take him up to his room, pop him in his LTD swaddle, tuck him in, give him his dummy, a kiss, and leave. He has white noise playing to muffle the noise in the house as we have a pretty small place. For his day naps I very rarely need to go in after that and he falls asleep quickly. When he goes down for the night we sometimes have to go in and give him a quick pat or sit with him for a bit before he'll go to sleep. Occasionally he just chats to himself for up to 2 hours so we just get him up, feed him and back to bed.
DD was never this easy, even though she was able to self settle she would always protest going to bed and would require a bit of a cuddle at bedtime. Even now at 3.5yrs it can be a bit of a process getting her to actually go to sleep (her mucking around in her room, not crying).
ETA - DS wasn't always like this, until we got his feeding sorted I would often cuddle him to sleep. He would only really feed to sleep at late night feeds. His sleep has been a hundred times better since I stopped breastfeeding (CMPI and suspected SPI, so he's on a specific formula for this. I'm certainly not saying formula is a magic sleep cure for all babies!).