Can USB (Data+) be read by arduino pins?

Hiya, I'm a bit of a newb to arduino.
Just wondering, if I attached the Data+ wire from a USB output would my arduino board be able to detect changes in it?

Might be able to "detect changes" but probably not fast enough to receive data. Note that Data+/Data- is a differential pair. You probably have to connect Data- to Ground to read a signal from Data+. If you want to receive data you will need something like a USB Host Shield.

I suspect that, because you are asking the question, the simple answer is NO.

USB is a very complex system that operates at much higher speed than an Atmega 328. The Atmega 32u4 in a Leonardo has special circuitry to deal with USB.