On a monocoque Italian car of this age, rust isn't a fault to find — it's a certainty to assess. The only real questions are where, how much, and how well any past repair was done. Start with the sills and their jacking points, because here rust is structural.
There is no rust-free car
The marque buyer's guides are blunt: sixty years on, there is essentially no such thing as a rust-free 750/101 Spider. Survivors either are rusty, or have been rusty and repaired — and the quality of that repair is what separates a sound car from a money pit. So the skill is not finding a perfect shell; it is judging what has been done to the one in front of you.
The sills and jacking points
The sills are structural on this monocoque, so corrosion here compromises the whole car. The classic tell is a collapsing or crumbling jacking point — if it is soft, folded, or has been plated over, treat the sills with suspicion. Look along the full length of each sill, inside and out, for filler, ripples, and fresh underseal that may be hiding a repair.
Reading a shell
Consistent panel gaps and doors that shut cleanly suggest a straight structure. Doors that drop or won't line up, and uneven or tapering gaps, can betray past structural rust or a repair that has since moved. Thick fresh underseal or sealer along the sills and floors is worth questioning — it can conceal as much as it protects.
Why it matters
Structural rust is the expensive kind. Cosmetic bubbling is one thing; the sills, jacking points and the structure around them are quite another, needing skilled cutting, fabrication and welding to put right. Factor it into the price, and on any car you are serious about, have it inspected on a ramp by someone who knows these Alfas.