if it's alive, it hasn't been designed (stuff like designer dog-breeding notwithstanding). rocks can be carved but if they're not on mount rushmore and don't have a signature or a plaque, they're probably not designed lol. if the hedge looks like a cat it's been trimmed but the hedge itself wasn't designed. if it's a chair, it was designed. birds design birds' nests but birds are not designed. beaver dams are harder to recognize but if you see beavers around you can guess.

g

Experience. You see some things in the world and then you acquire the knowledge that some things are designed by humans (or animals) and others aren't.

when you discuss something like a watch, or a house, then we recognize that it was built by people because we grow up knowing it was built by people. we don't "recognize" that it was designed. we just know. when we discuss things outside of our experience, we recognize elements of it that are consistent with design we are familiar with.

since there is no possible way this post isn't about religion, the way we recognize something is "designed" is that it conflicts with our experience of things that aren't. in order for that to be true, we must acknowledge knowing that natural things do not actually have the same elements as designed things. if we were to accept that we think things in nature are "designed", we would not be able to distinguish things that are designed from them.

I don't understand.
Do you mean design as in "intelligent design" theories?

by being taught the mechanics of design. infants actually don't recognize it, and primitive people have been known to confuse modern technology with magic on numerous occasions, because they were not exposed to the elements of design common to the technology.