as Detee has said, there are ways to determine whether something was competently designed.
Unlike Behe I can think of several features that are highly indicative of either an evolved system or one that has been competently designed.
1) Designers can correct their mistakes
Given that Behe accepts common descent of humans and chimps, why would the designer not fix the appendix so it didn't burst, once this " obviously not-so omniscient" designer noticed the first hominid case of appendicitis?
2) Designers can reuse aspects of their designs
There are several animals that have additional eyes (e.g Notostraca). Their "third-eye" is different to their two compound-eyes, which is perfectly consistent with an evolved system, where there is no "defined function" the system just does what it does. However most competent designers, particularly those competent enough to desing one type of eye, would simply reuse that eye design for any additional ones.
Similarly, convergent evolution, where many organisms in similar environments demonstrate similar traits, is further evidence against a designer acting like this. Why are there so many different plants that produce fruit? Why are there so many different parts of the flower that turn into fruit (either true-fruit, or false fruit)? This is easy to explain if they evolved separately, but hard if you posit a designer attempting different solutions, to the same problem.
3) Competent designers don't get something right, then get it wrong later without correcting it
The octopus-retina lacks many of the drawbacks of the mammalian retina, which came later (or the same time if you are a YEC, which Behe isn't). Any human with the intelligence to design an eye would also spot the flaw in the mammalian "design".
4)Evolved systems can only get "information" from their ancestors
If a traits evolved separately in different organisms, the genes that express these traits should have different sequences, whilst if they were designed, they could be quite likely to have the same genetic sequence. Should anyone find this, when there hasn't been lateral gene transfer, then this would require some explanation. But (as the world's entire news media hasn't trumpeted this discovery) this hasn't been found.
5) Evolved systems are quite likely to "throw away" traits that are no longer advantageous
Why are vestigial organs vestigial, and neither fully working nor non-existent, which would look more elegant. For example either Basilosaurus, or modern whales have the more aesthetic rear leg "solution". Why did Basilosaurus have vestigial legs?
I am sure there are more but that should show why I disagree with Behe's admission...ETA that one couldn't test for ID.