We get by on a combination of matching patterns (any pointer cast gets a lot of scrutiny, for example), compiler warnings, tools like UBSan, debugging when things go wrong, and sheer dumb luck.

Having an understanding of how the code gets transformed into machine code helps. For this case, there's the basic idea that `a++` will boil down to three basic conceptual operations: fetch, add, and store, and those can be potentially interleaved with other parts of the statement. In something like `a++ + ++b` the interleaving doesn't affect the outcome no matter how it's done. In `a++ + ++b` the interleaving can affect the outcome, and that's your sign that something might be wrong.

Any memory safety issue in C code had to involve UB at some point. And you can see how prevalent those are, and deduce how not-particularly-great we are at keeping track of UB.