You wouldn't know __ if it __ed you is a common idiom scheme (or snowclone) which means that you wouldn't know or recognize something even if was very obvious.
The idea is that if something hit you in the face, bit you on the ass, or was such that you fell over it, then if would be pretty obvious to you what the thing was. But the sentence declares that even then, you are so oblivious that you still wouldn't know what the thing was.
Here are yet more variants:
- you wouldn't know __ if it was right in front of you
- you wouldn't know __ if it was staring you in the face
- you wouldn't know __ if it was right under your nose
You can embellish this kind of structure to make it more hyperbolic and colorful, as in your example of:
- you wouldn't know __ if it walked up and bit your nose off
This might be viewed as a play on the original structure as well as the expression be right under your nose, which means "to be in a place that you can clearly see."