My daughter has clear signals for when it's nap time. Or at least, they're clear to me, since I spend a lot of time with her.
Four months ago, when I was desperate for a good night's rest (and didn't get one for another three and a half months) I read a few books (and a few articles in magazines and an embarrassing amount of on-line articles and message boards) on baby sleep habits. [My favorite of these was the message board where mother after mother after mother told how great it was that their child slept through the night at two months, so it was easy to get kids to do this, until the one honest lady said: If your kid sleeps through the night that young it's the kid, not you the mother, you have done nothing, and you're just lucky. I wanted to be friends with her.]
Some advice made sense, some applied, some did not, the usual routine with parenting advice. The one message I got loud and clear, and felt made sense, (and probably believed because it was from the book that all the other books and articles cited) was that babies need a routine to help them calm down and realize its bed time. Everytime you put them down they should have this routine, although you can different routines for naptime and bedtime.
Our bedtime routine was officially instituted, and while I don't believe it settles her down, it is a nice family routine, including family prayer. As for a nap routine, I sing her a song and put her down. Until recently. Once she finally convinces me that she is ready to nap (often sooner than I am expecting) we go into her room, and I try to sing her a song. In the past few weeks she has gotten agitated at me, and starts to wiggle as if trying to get into her crib. She has no patience for the song because she wants to go to sleep. Today she actually started crying when I started singing. I think I'm dropping the song before the nap. She doesn't seem to care for it anyway.
It's a blessing to have a baby who goes down so easily for a nap. But, it has everything to do with our home and her crib. When we're out on church visits, or camp, or anywhere else, she just goes and goes until she collapses from exhaustion (sans melt-down, thank goodness), like last Sunday when she finally fell asleep sitting on my lap during a meeting. What a good sleeper.