My filly, who has a tendency to go catatonic and then blow up massively, when she ISN'T catatonic is not difficult to read. Subtle, but not difficult. Worried eyes, tight mouth [she has a really strange expression actually, almost like a human's disgusted look], tight ears. Head goes up. Feet plant, tail clamps, she is quite textbook.
When she IS catatonic, I cannot read her. She seems totally calm outwardly. The best description is that she goes to her happy place to try to control the uncomfortable feelings, and there's not a single sign that I can see that she is anything other than completely ok. Until it gets too much and she blows up. Literally out of nowhere.
Slowly I am learning how to keep her from going to her happy place, and she is learning that it's ok to show me when she's scared. Everyone is happier when she displays her fear because I can know when to push the point and when to back off for a moment and let her settle down. She is no less afraid, but her person doesn't keep pushing if she says she's too scared. We have fewer blowups since it clicked in her mind that she actually CAN show her fear [it is still inconsistent] because I'm not pushing her over the edge.
I find my gelding, OTOH, to be rather more difficult to read. He is naturally a very curious horse and this makes him rather 'looky' as a baseline. Typically the first I know of something having scared him is my horse teleporting a few feet sideways, then spinning and trying to gallop off. That or he suddenly refuses to move forward no matter what I do. It depends on where the scary thing is.