my two are VERY different, DD1 couldnt stand being too dressed, in summer (when she was only a few months old, and now that she is 3) she would rather sleep in undies (or nappy when she was little) and a vest most nights, and still kicks toe covers off because she is hot!
DD2 is much colder, even on the hottest night we had the other day, she was in long sleves and legs.
Ive always had both mine in bed with me though, so its easier to gauge the temp and warmer so they require less.
like a PP said, you just know. i read that they require about one extra layer of clothes than your a re wearing.