On the 18th of December when we get our results, will the scaling be such that I'll be able to look at my marks and automatically be able to determine which two of my units have not been counted
Not strictly, because the HSC marks that are reported will be different to the marks used to calculate your UAI.
Your UAI is determined by UAC and has it's own scaling process, using your
raw HSC mark. On the 18th you will only see your
aligned HSC mark, which "scales twice" in effect - as it uses your assessment mark (moderated, and then aligned) combined with another alignment process - taking your raw exam mark, aligning it, and combining the two to get your
aligned HSC mark (which differs to your scaled mark, as used by UAC).
You will have to wait until you receive your UAI to tell.
Basically, if I see a bad HSC mark on the 18th (my worst), is it safe to be able to tell myself that it won't be counted?
No, I don't think you can "simply" do that, on inspection.