How a developer can tell if the system is sick just by taking a look at the input and the output? If you’d like to know that (plus you like standup comedy), come and see! The treatment is safe ;-) One approach to get familiar with a system is a long and intense reading the manual (and to run an eye over the source code sometimes). Another approach is to take a deep dive into the data model (often in the DB) and to surf the user interface.
“The best sorting algorithm is quick sort." “Indexes make DB faster." “Data should be sorted using ORDER BY." “Composition - good; inheritance - not good." “Windows is an operating system." “You must have transactions in your DB." “Java is slow." “Don’t eat yellow snow." “You shall not self-sign your certificates." “Interrupt in Java is broken.” The IT world is full of mantras/revealed truth, passed (often in oral tradition) among developer tribes.