Remembering - that is the big question unfortunately. It is somewhat complicated by there being more than one known route to autism. I have had docs say they would like to see more work into identifying susceptible groups but I'm not sure it's the sort of work that could be done at the moment tbh
One has already been mentioned - an underlying mitochondrial disorder - I think most doctors now would agree you would need to be careful in those cases, but there isn't an easy way to test for that and the type of disorder involved doesn't really give obvious symptoms. You could probably find out more if you contact Kennedy Krieger in the States, I've always found them approachable & they work on this sort of thing.
Another time I personally would be careful would be if the mother has had a viral infection during very early pregnancy. There are a few promising models that have been investigating that & some (not all) include vaccinations as a potential later hit triggering regression in an already primed immune systwm. Unfortunately they tend not to publish that bit, so it's hard to know what current thinking on that is, although will happily discuss it in person or during conference papers. I doubt they'd go as far as advising not to vaccinate though (esp as in that model any virus is a risk).
In the Wakefield style regression with subsequent GI issues who knows, officially there's no such thing, although I do know gastroenterologists who have told parents with eg Crohns in the family to be careful. I don't believe there's evidence for that as such though.
Knowing more than one family where two children have regressed post MMR (with very extreme reactions) I'd be careful after one sibling reaction tbh. There's absolutely no evidence for that though obviously (although it is the sort of case where senior clinicians are usually sympathetic & would happily talk it through).
Am happy to give further info about our own personal decisions (which weren't really based on any of the above) if you PM me. In terms of ds1 - there is a lot with hindsight that I would have done differently but I'm not sure how many warning signs there were (some, but not many).