To know the damage level on any unit, right-click on it and look near the top of the data column, there should be a "damage" entry if it isn't null.
Suppression only tells you about, well, suppression, i.e. the morale state of your unit. As Andy said, you can rally suppression, but not repair damage, so the two are different things.
Thanks Plasma, I was aware of that entry though I didn´t pay much atention to it, in this case it indicates a damage of "2" for my hinds, the same for a inmobilised tank, do you now what´s the scale?
No idea if there's such thing as a scale to speak of.
AFAIK for infantry one damage is one man disabled, I guess you can apply the same at least to soft-skin vehicles and crewed weapons. For AFVs, either it's about this crew vvalue or survivability (i.e. a vehicle can take as much damage as it has survivability).
Airplane and helos are a special case, the amount of damage it can take is set by its "front hull steel" armor value IIRC, where different value are just on an abstract scale of this plane is more resilient than that one.