There’s some things you look for that are difficult to describe to someone who hasn’t seen it before. That’s part of why experience is so valuable in Emergency Medicine, and it’s not uncommon to put your best nurses out in triage. People will do this kinda twitchy/wilting/loss of focus/change in pallor/change in posture right before they go down. I don’t have a good way to describe it, and it might be easier to draw even, because it really is a body language thing and the general appearance of the patient that can inform your decision making.