Although signs of evolution are to be found everywhere across taxa, it sometimes seems difficult to discern whether a trait is a retained accident or an adaptation. It also seems like there is a temptation in people to come up with just-so stories to explain traits as adaptations. What is an effective strategy for coming up with tests to determine whether a trait is an adaptation or not?
See if the trait gives any significant advantages as compared to absences of trait. Also if the trait is present in almost all organisms and no variation is found, then see if it has any use. Like appendix is found in almost every human; although it can be surgically removed, normally it is present, but is useless in humans.