You can’t tell if expanded polystyrene has structrual damage. If it really did save your head, it’s time to replace it. The way they absorb the energy, like seat belts, child seats and crumple zones, is by damaging their internal structure I believe – and you’re meant to replace any of those devices after they have been used.
Polystyrene also goes off over time – though I’m not sure if that’s due to time or to light (possibly UV) so old helmets ain’t as effective.
I’ve put myself in hospital for a week with concussion – I wouldn’t recommend it, particularly as with a lot of people on here your livelihood depends on being able to use you brain.
Broken bones on the other hand are fine; so save money by not buying body armour and get replace your hat when it needs it.