The thing about evidence for design in nature is that it looks kind of obvious intuitively, but it’s very, very hard to prove with any form of scientific rigour. Part of the problem is that we don’t have any objective measurable criteria for what would constitute evidence for design. ID proponents have come up with concepts such as irreducible complexity, but they tend to jump the gun and claim that structures such as the bacterial flagellum are irreducibly complex when in reality they are not.
Even when it comes to things designed by humans, there are some things where design is indistinguishable from randomness. Cryptography is one example here: well-designed cryptographic algorithms result in files that contain vast amounts of information (trade secrets, personal information etc) but without the decryption keys and an understanding of the protocols used to encode the information, they are indistinguishable from random noise.
As far as Romans 1:19-20 is concerned, the way I understand it is that it tells us that what we see in nature tells us something about the processes that God used to create everything, and that these details are clear and unambiguous even to unbelievers. In other words, that what we see in nature is an accurate reflection of reality.
For example, we can see from the expansion of the universe that it had a beginning. Back in the mid 20th century there were two competing theories for the origin of the universe: the Big Bang theory, which said that it had a beginning, and the Steady State theory, which said that it didn’t. A lot of atheists preferred the Steady State theory (the term “Big Bang” was actually coined as a term of derision by Fred Hoyle, who hated the idea) but the Big Bang won the day with the discovery of the cosmic microwave background by Penzias and Wilson in 1965.
Evolution is a subject that is often misunderstood. It’s quite clear from the evidence that God used evolutionary processes – descent with modification over billions of years – to create humanity the way that we are today. But this doesn’t tell us that God doesn’t exist. Rather, it tells us that we’re not as important as we like to think that we are. It takes a lot of humility to accept that we are related to chimpanzees, and one fairly common reason why creationists object to it is that it offends their pride. (I say this because this was a mistake that I made myself.) But these verses in Ecclesiastes 3:18-21 should give us pause to reflect here:
I also said to myself, “As for humans, God tests them so that they may see that they are like the animals. Surely the fate of human beings is like that of the animals; the same fate awaits them both: As one dies, so dies the other. All have the same breath; humans have no advantage over animals. Everything is meaningless. All go to the same place; all come from dust, and to dust all return. Who knows if the human spirit rises upward and if the spirit of the animal goes down into the earth?”
So basically, I view Romans 1:19-20 as not just telling us about evidence for creation, but about knowing what our place is in it. It’s really a call to humility and not to think that you can just get away with behaving however you like.