One way of checking if they are too hot is it their feet and the back of their neck are clammy and sweaty.

Atm I have Aidyn in his 1.0 tog grobag on some nights with a pj top and pants underneath (no singlet or socks). But on some nights he just gets too hot even in his 1.0tog grobag, so I put him in flanalette pj's with a singlet and socks (and no blanket), and he will be fine like that for the whole night and not get too cold. He is a pretty hot sleeper though (like his Dad!)