I can tell if mine are too cold. They will curl in in a tiny ball on a blanket or pillow and not be very active. Sometimes they hide in their crate (it's fabric and has warm fluffy pillows) or will want to snuggle close to me or sit on my lap all day. Usually I am cold before they are. If I can't turn the heat up (if my husband is warm
) I will put t-shirts or sweatshirts on them. They have a harder time now, adjusting to the cold after a hot summer. I bundle them up in coats and boots for the first snow but by January they are out there naked unless it's below 0.