If they are good at sex
DNA unless coloring different for that species between males/females.
because they look different
The koala has an acutely sensitive nose, which enables it to detect subtle scent differences between different species of gum trees.
There is no one specific way to tell the difference between male and female fish. Different species have different ques, and in the vast majority of fish, there is really no reliable external way to tell a fish's sex.
There are significant differences between different species of fish, in terms of their breeding. You must tell us what kind of fish you have, before we can advise you.
Yes, different species of turtles have many different defining characteristics. The real question here is, how does one determine the difference by touch?
That depends entirely on the species.
different organism have dna with nucleotide base in a different order
dichotomous key
Back down the line, the species came from a common ancestor.
Each fish is different to tell the gender. Usually, the female is the larger one, and there are color differences, varying from species to species. Consult a field guide.
Sometimes it is very hard to distinguish between a species where there is a lot of variation between that of a variation that is actually two separate species. Many times scientist disagree on how to tell these two things apart.