How can I view the SS line on an oscilliscope (Due)?

According to the Arduino SPI documentation, the Due uses pins 4, 10, and 52 for for chip select. When I measured the three pins, only 4 was high. However, 4 remains high, even during transmission, and never goes logic-low. I tried dictating the chip select pin (e.g. by using SPI.begin(10)), and that would set the pin but then transmission would not even work.

I only use direct register programming for SPI code and the SPI peripheral handles automatically the Chip select control line (turned on and off by hardware) unlike the SPI library which does not work this way (I guess), you have to handle manually the chip select control line. You can see that in the SPI example sketches which come with the library.

Conclusion: with this library, you can choose any control pin you want as a chip select since you have to turn it on and off manually to select the SPI slave.