Interpreting your question the best I can:

There are usually patterns that you match the code you're reading to. For example, the simple pattern of int f(int x, int y, int len, int *z);

and you immediately think "oh ok, they're doing something with x and y, and z is the out parameter. The fn is also likely going to allocate". Usually it matches what you see in the function body and it's all good. In (hopefully) rare cases you view it with such a lens and something doesn't make sense and you have to scrutinise the code for what it's actually doing. This is all without variable names. Proper variable names makes it much more straightforward to read e.g calling the function vec_add