How do I know if my dogs actually like being pet?

I've been trying to learn about dog body language recently, as I want to understand my dogs better.

Often, when I sit on the sofa with a blanket on my legs, one or both of my two shih tzus will jump up and sleep on them. It's adorable, thus my instinct is to pet them.

They say that a dog shows that they like being pet when they actively try to re-initiate contact, but when my dogs sit on my legs, they rarely do. Would that be because they're sleepy, or because they don't actually want me to touch them?

Furthermore, if it turns out they don't particularly like being pet, how do I show affection?