How does someone act if they like someone but doesn't want to share it openly. I'd like to see what people think.

Everyone expresses their feelings differently.

Instead of trying to figure out what's going on in the other person's mind, why not focus on getting to know the other person. Once you're comfortable with each other, you can ask directly.

That way, you don't have to play mind games.

As Iwish said above different people display their affections differently. However there are a few things that you can look for that may indicate that someone likes or is interested in you.

They stare or glance at you a lot
They smile at you
They try to converse with you
They touch you lightly when speaking
Etc etc

Now Love , that's a totally different thing , that comes after A LOT of liking.

There is no way to tell if they really don't show it obviousl or if they don't tell you

Different people act differently.
I would assume they would only react to the person when no one else is looking and that is a bit suspect.
Perhaps I just don't understand your question

Why don't you want to share your feelings openly? What are you hiding them for?