This is an interesting point. Where something suits its function, for example an eye, how can we tell if it was designed or evolved? Both design and evolution predict that eyes will function correctly as eyes.
We can resolve the question by looking at things that do
not function correctly. Here evolution and design predict different things. Evolution allows the existence of non-working parts providing they are non-essential. Non working parts are much more difficult to explain from a design point of view. If they are non-essential then why are they there in the first place? If you are going to have them anyway then why don’t they work?
Almost every animal can make its own vitamin C. Two exceptions are primates (including us) and Guinea Pigs. Humans like other primates can get scurvy if they do not eat enough vitamin C. Since the normal primate diet contains plenty of fresh food this is not usually a problem. The broken vitamin C production system is an example of a non-working non-essential system.
When we started sequencing genes, we found that the DNA errors that broke the vitamin C system are shared between all the primates. Our DNA has what was once a functioning gene but it does not work because of errors. This pseudo-gene matches a functioning gene in non-primates that can still make vitamin C. Identical errors are present in all primates, including humans. Guinea Pigs have a different set of errors. For evolution the obvious explanation is that as primates evolved from their common ancestor the same error was passed on to all descendants, including ourselves. There was one initial error which was passed on. Guinea Pigs have a different error because they are descended from a different ancestor which had a different initial error.
Design finds it difficult to explain this. Having made an error, why did the designer copy the exact same error into all other primates, but use a different error in Guinea Pigs? Is that intelligent design?
In this case the evidence can distinguish between the actions of an intelligent designer and those of evolution.
For more details and other examples see
Plagiarized Errors and Molecular Genetics.
rossum