For checking ubercharge: You could check the condition's inflictor with low-level memory reads, but that just narrows the effect down to a client.
I have accessors for those, but they're not all that safe -- probably should get it set up as a proper library at some point.
A pretty basic way to check would be to use Pelipoika's
GetHealerByIndex function stock to iterate over a target's healers, then check if they have the correct medigun out with charge currently being drained.
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For checking invulnerability: TF2 itself implements
CTFPlayerShared::IsInvulnerable(), and all it does is check if the player has
TF_COND_INVULNERABLE,
TF_COND_INVULNERABLE_USER_BUFF,
TF_COND_INVULNERABLE_HIDE_UNLESS_DAMAGED, or
TF_COND_INVULNERABLE_CARD_EFFECT conditions set.